#
コールバック数

コールバックは、PyTorch のトレーニング ループの動作をカスタマイズできるオブジェクトです。 トレーニング ループを検査できる Trainer (この機能は TensorFlow にはまだ実装されていません) 状態を確認し (進捗レポート、TensorBoard または他の ML プラットフォームへのログ記録など)、決定を下します (初期段階など)。 停止中）。

コールバックは、返される TrainerControl オブジェクトを除けば、「読み取り専用」のコード部分です。 トレーニング ループ内では何も変更できません。トレーニング ループの変更が必要なカスタマイズの場合は、次のことを行う必要があります。 Trainer をサブクラス化し、必要なメソッドをオーバーライドします (例については、trainer を参照してください)。

デフォルトでは、`TrainingArguments.report_to` は `"all"` に設定されているため、Trainer は次のコールバックを使用します。

- DefaultFlowCallback は、ログ記録、保存、評価のデフォルトの動作を処理します。
- PrinterCallback または ProgressCallback で進行状況を表示し、 ログ (最初のログは、TrainingArguments を通じて tqdm を非アクティブ化する場合に使用され、そうでない場合に使用されます) 2番目です)。
- TensorBoardCallback (PyTorch >= 1.4 を介して) tensorboard にアクセスできる場合 またはテンソルボードX）。
- WandbCallback wandb がインストールされている場合。
- CometCallback comet_ml がインストールされている場合。
- mlflow がインストールされている場合は MLflowCallback。
- NeptuneCallback neptune がインストールされている場合。
- AzureMLCallback azureml-sdk の場合 インストールされています。
- CodeCarbonCallback codecarbon の場合 インストールされています。
- ClearMLCallback clearml がインストールされている場合。
- DagsHubCallback dagshub がインストールされている場合。
- FlyteCallback flyte がインストールされている場合。
- DVCLiveCallback dvclive がインストールされている場合。
- SwanLabCallback swanlab がインストールされている場合。

パッケージがインストールされているが、付随する統合を使用したくない場合は、`TrainingArguments.report_to` を、使用したい統合のみのリストに変更できます (例: `["azure_ml", "wandb"]`) 。

コールバックを実装するメインクラスは TrainerCallback です。それは、 TrainingArguments は Trainer をインスタンス化するために使用され、それにアクセスできます。 TrainerState を介してトレーナーの内部状態を取得し、トレーニング ループ上でいくつかのアクションを実行できます。 TrainerControl。

##
利用可能なコールバック

ライブラリで利用可能な TrainerCallback のリストは次のとおりです。

###
class transformers.integrations.CometCallback

< source >

( )

A TrainerCallback that sends the logs to Comet ML.

####
setup

< source >

( args state model )

Setup the optional Comet integration.

Environment:

- COMET_MODE (`str`, optional, default to `get_or_create`): Control whether to create and log to a new Comet experiment or append to an existing experiment. It accepts the following values:
  - `get_or_create`: Decides automatically depending if `COMET_EXPERIMENT_KEY` is set and whether an Experiment with that key already exists or not.
  - `create`: Always create a new Comet Experiment.
  - `get`: Always try to append to an Existing Comet Experiment. Requires `COMET_EXPERIMENT_KEY` to be set.
  - `ONLINE`: deprecated, used to create an online Experiment. Use `COMET_START_ONLINE=1` instead.
  - `OFFLINE`: deprecated, used to created an offline Experiment. Use `COMET_START_ONLINE=0` instead.
  - `DISABLED`: deprecated, used to disable Comet logging. Use the `--report_to` flag to control the integrations used for logging result instead.
- COMET_PROJECT_NAME (`str`, optional): Comet project name for experiments.
- COMET_LOG_ASSETS (`str`, optional, defaults to `TRUE`): Whether or not to log training assets (tf event logs, checkpoints, etc), to Comet. Can be `TRUE`, or `FALSE`.

For a number of configurable items in the environment, see here.

###
class transformers.DefaultFlowCallback

< source >

( )

A TrainerCallback that handles the default flow of the training loop for logs, evaluation and checkpoints.

###
class transformers.PrinterCallback

< source >

( )

A bare TrainerCallback that just prints the logs.

###
class transformers.ProgressCallback

< source >

( max_str_len: int = 100 )

A TrainerCallback that displays the progress of training or evaluation. You can modify `max_str_len` to control how long strings are truncated when logging.

###
class transformers.EarlyStoppingCallback

< source >

( early_stopping_patience: int = 1 early_stopping_threshold: typing.Optional[float] = 0.0 )

Parameters

- early_stopping_patience (`int`) — Use with `metric_for_best_model` to stop training when the specified metric worsens for `early_stopping_patience` evaluation calls.
- early_stopping_threshold(`float`, optional) — Use with TrainingArguments `metric_for_best_model` and `early_stopping_patience` to denote how much the specified metric must improve to satisfy early stopping conditions. `

A TrainerCallback that handles early stopping.

This callback depends on TrainingArguments argument load_best_model_at_end functionality to set best_metric in TrainerState. Note that if the TrainingArguments argument save_steps differs from eval_steps, the early stopping will not occur until the next save step.

###
class transformers.integrations.TensorBoardCallback

< source >

( tb_writer = None )

Parameters

- tb_writer (`SummaryWriter`, optional) — The writer to use. Will instantiate one if not set.

A TrainerCallback that sends the logs to TensorBoard.

###
class transformers.integrations.WandbCallback

< source >

( )

A TrainerCallback that logs metrics, media, model checkpoints to Weight and Biases.

####
setup

< source >

( args state model **kwargs )

Setup the optional Weights & Biases (wandb) integration.

One can subclass and override this method to customize the setup if needed. Find more information here. You can also override the following environment variables:

Environment:

- WANDB_LOG_MODEL (`str`, optional, defaults to `"false"`): Whether to log model and checkpoints during training. Can be `"end"`, `"checkpoint"` or `"false"`. If set to `"end"`, the model will be uploaded at the end of training. If set to `"checkpoint"`, the checkpoint will be uploaded every `args.save_steps` . If set to `"false"`, the model will not be uploaded. Use along with `load_best_model_at_end()` to upload best model.

  Deprecated in 5.0

  Setting `WANDB_LOG_MODEL` as `bool` will be deprecated in version 5 of 🤗 Transformers.

- WANDB_WATCH (`str`, optional defaults to `"false"`): Can be `"gradients"`, `"all"`, `"parameters"`, or `"false"`. Set to `"all"` to log gradients and parameters.

- WANDB_PROJECT (`str`, optional, defaults to `"huggingface"`): Set this to a custom string to store results in a different project.

- WANDB_DISABLED (`bool`, optional, defaults to `False`): Whether to disable wandb entirely. Set `WANDB_DISABLED=true` to disable.

###
class transformers.integrations.MLflowCallback

< source >

( )

A TrainerCallback that sends the logs to MLflow. Can be disabled by setting environment variable `DISABLE_MLFLOW_INTEGRATION = TRUE`.

####
setup

< source >

( args state model )

Setup the optional MLflow integration.

Environment:

- HF_MLFLOW_LOG_ARTIFACTS (`str`, optional): Whether to use MLflow `.log_artifact()` facility to log artifacts. This only makes sense if logging to a remote server, e.g. s3 or GCS. If set to `True` or 1, will copy each saved checkpoint on each save in TrainingArguments’s `output_dir` to the local or remote artifact storage. Using it without a remote storage will just copy the files to your artifact location.
- MLFLOW_TRACKING_URI (`str`, optional): Whether to store runs at a specific path or remote server. Unset by default, which skips setting the tracking URI entirely.
- MLFLOW_EXPERIMENT_NAME (`str`, optional, defaults to `None`): Whether to use an MLflow experiment_name under which to launch the run. Default to `None` which will point to the `Default` experiment in MLflow. Otherwise, it is a case sensitive name of the experiment to be activated. If an experiment with this name does not exist, a new experiment with this name is created.
- MLFLOW_TAGS (`str`, optional): A string dump of a dictionary of key/value pair to be added to the MLflow run as tags. Example: `os.environ['MLFLOW_TAGS']='{"release.candidate": "RC1", "release.version": "2.2.0"}'`.
- MLFLOW_NESTED_RUN (`str`, optional): Whether to use MLflow nested runs. If set to `True` or 1, will create a nested run inside the current run.
- MLFLOW_RUN_ID (`str`, optional): Allow to reattach to an existing run which can be useful when resuming training from a checkpoint. When `MLFLOW_RUN_ID` environment variable is set, `start_run` attempts to resume a run with the specified run ID and other parameters are ignored.
- MLFLOW_FLATTEN_PARAMS (`str`, optional, defaults to `False`): Whether to flatten the parameters dictionary before logging.
- MLFLOW_MAX_LOG_PARAMS (`int`, optional): Set the maximum number of parameters to log in the run.

###
class transformers.integrations.AzureMLCallback

< source >

( azureml_run = None )

A TrainerCallback that sends the logs to AzureML.

###
class transformers.integrations.CodeCarbonCallback

< source >

( )

A TrainerCallback that tracks the CO2 emission of training.

###
class transformers.integrations.NeptuneCallback

< source >

( api_token: typing.Optional[str] = None project: typing.Optional[str] = None name: typing.Optional[str] = None base_namespace: str = 'finetuning' run = None log_parameters: bool = True log_checkpoints: typing.Optional[str] = None **neptune_run_kwargs )

Parameters

- api_token (`str`, optional) — Neptune API token obtained upon registration. You can leave this argument out if you have saved your token to the `NEPTUNE_API_TOKEN` environment variable (strongly recommended). See full setup instructions in the docs.
- project (`str`, optional) — Name of an existing Neptune project, in the form “workspace-name/project-name”. You can find and copy the name in Neptune from the project settings -> Properties. If None (default), the value of the `NEPTUNE_PROJECT` environment variable is used.
- name (`str`, optional) — Custom name for the run.
- base_namespace (`str`, optional, defaults to “finetuning”) — In the Neptune run, the root namespace that will contain all of the metadata logged by the callback.
- log_parameters (`bool`, optional, defaults to `True`) — If True, logs all Trainer arguments and model parameters provided by the Trainer.
- log_checkpoints (`str`, optional) — If “same”, uploads checkpoints whenever they are saved by the Trainer. If “last”, uploads only the most recently saved checkpoint. If “best”, uploads the best checkpoint (among the ones saved by the Trainer). If `None`, does not upload checkpoints.
- run (`Run`, optional) — Pass a Neptune run object if you want to continue logging to an existing run. Read more about resuming runs in the docs.
- **neptune_run_kwargs (optional) — Additional keyword arguments to be passed directly to the `neptune.init_run()` function when a new run is created.

TrainerCallback that sends the logs to Neptune.

For instructions and examples, see the Transformers integration guide in the Neptune documentation.

###
class transformers.integrations.ClearMLCallback

< source >

( )

A TrainerCallback that sends the logs to ClearML.

Environment:

- CLEARML_PROJECT (`str`, optional, defaults to `HuggingFace Transformers`): ClearML project name.
- CLEARML_TASK (`str`, optional, defaults to `Trainer`): ClearML task name.
- CLEARML_LOG_MODEL (`bool`, optional, defaults to `False`): Whether to log models as artifacts during training.

###
class transformers.integrations.DagsHubCallback

< source >

( )

A TrainerCallback that logs to DagsHub. Extends `MLflowCallback`

####
setup

< source >

( *args **kwargs )

Setup the DagsHub’s Logging integration.

Environment:

- HF_DAGSHUB_LOG_ARTIFACTS (`str`, optional): Whether to save the data and model artifacts for the experiment. Default to `False`.

###
class transformers.integrations.FlyteCallback

< source >

( save_log_history: bool = True sync_checkpoints: bool = True )

Parameters

- save_log_history (`bool`, optional, defaults to `True`) — When set to True, the training logs are saved as a Flyte Deck.
- sync_checkpoints (`bool`, optional, defaults to `True`) — When set to True, checkpoints are synced with Flyte and can be used to resume training in the case of an interruption.

A TrainerCallback that sends the logs to Flyte. NOTE: This callback only works within a Flyte task.

Example:

    # Note: This example skips over some setup steps for brevity.
    from flytekit import current_context, task

    @task
    def train_hf_transformer():
        cp = current_context().checkpoint
        trainer = Trainer(..., callbacks=[FlyteCallback()])
        output = trainer.train(resume_from_checkpoint=cp.restore())

###
class transformers.integrations.DVCLiveCallback

< source >

( live: typing.Optional[typing.Any] = None log_model: typing.Union[typing.Literal['all'], bool, NoneType] = None **kwargs )

Parameters

- live (`dvclive.Live`, optional, defaults to `None`) — Optional Live instance. If None, a new instance will be created using **kwargs.
- log_model (Union[Literal[“all”], bool], optional, defaults to `None`) — Whether to use `dvclive.Live.log_artifact()` to log checkpoints created by Trainer. If set to `True`, the final checkpoint is logged at the end of training. If set to `"all"`, the entire TrainingArguments’s `output_dir` is logged at each checkpoint.

A TrainerCallback that sends the logs to DVCLive.

Use the environment variables below in `setup` to configure the integration. To customize this callback beyond those environment variables, see here.

####
setup

< source >

( args state model )

Setup the optional DVCLive integration. To customize this callback beyond the environment variables below, see here.

Environment:

- HF_DVCLIVE_LOG_MODEL (`str`, optional): Whether to use `dvclive.Live.log_artifact()` to log checkpoints created by Trainer. If set to `True` or 1, the final checkpoint is logged at the end of training. If set to `all`, the entire TrainingArguments’s `output_dir` is logged at each checkpoint.

###
class transformers.integrations.SwanLabCallback

< source >

( )

A TrainerCallback that logs metrics, media, model checkpoints to SwanLab.

####
setup

< source >

( args state model **kwargs )

Setup the optional SwanLab (swanlab) integration.

One can subclass and override this method to customize the setup if needed. Find more information here.

You can also override the following environment variables. Find more information about environment variables here

Environment:

- SWANLAB_API_KEY (`str`, optional, defaults to `None`): Cloud API Key. During login, this environment variable is checked first. If it doesn’t exist, the system checks if the user is already logged in. If not, the login process is initiated.

  - If a string is passed to the login interface, this environment variable is ignored.
  - If the user is already logged in, this environment variable takes precedence over locally stored login information.

- SWANLAB_PROJECT (`str`, optional, defaults to `None`): Set this to a custom string to store results in a different project. If not specified, the name of the current running directory is used.

- SWANLAB_LOG_DIR (`str`, optional, defaults to `swanlog`): This environment variable specifies the storage path for log files when running in local mode. By default, logs are saved in a folder named swanlog under the working directory.

- SWANLAB_MODE (`Literal["local", "cloud", "disabled"]`, optional, defaults to `cloud`): SwanLab’s parsing mode, which involves callbacks registered by the operator. Currently, there are three modes: local, cloud, and disabled. Note: Case-sensitive. Find more information here

- SWANLAB_LOG_MODEL (`str`, optional, defaults to `None`): SwanLab does not currently support the save mode functionality.This feature will be available in a future release

- SWANLAB_WEB_HOST (`str`, optional, defaults to `None`): Web address for the SwanLab cloud environment for private version (its free)

- SWANLAB_API_HOST (`str`, optional, defaults to `None`): API address for the SwanLab cloud environment for private version (its free)

##
TrainerCallback

###
class transformers.TrainerCallback

< source >

( )

Parameters

- args (TrainingArguments) — The training arguments used to instantiate the Trainer.
- state (TrainerState) — The current state of the Trainer.
- control (TrainerControl) — The object that is returned to the Trainer and can be used to make some decisions.
- model (PreTrainedModel or `torch.nn.Module`) — The model being trained.
- tokenizer (PreTrainedTokenizer) — The tokenizer used for encoding the data. This is deprecated in favour of `processing_class`.
- processing_class ([`PreTrainedTokenizer` or `BaseImageProcessor` or `ProcessorMixin` or `FeatureExtractionMixin`]) — The processing class used for encoding the data. Can be a tokenizer, a processor, an image processor or a feature extractor.
- optimizer (`torch.optim.Optimizer`) — The optimizer used for the training steps.
- lr_scheduler (`torch.optim.lr_scheduler.LambdaLR`) — The scheduler used for setting the learning rate.
- train_dataloader (`torch.utils.data.DataLoader`, optional) — The current dataloader used for training.
- eval_dataloader (`torch.utils.data.DataLoader`, optional) — The current dataloader used for evaluation.
- metrics (`Dict[str, float]`) — The metrics computed by the last evaluation phase.

  Those are only accessible in the event `on_evaluate`.

- logs (`Dict[str, float]`) — The values to log.

  Those are only accessible in the event `on_log`.

A class for objects that will inspect the state of the training loop at some events and take some decisions. At each of those events the following arguments are available:

The `control` object is the only one that can be changed by the callback, in which case the event that changes it should return the modified version.

The argument `args`, `state` and `control` are positionals for all events, all the others are grouped in `kwargs`. You can unpack the ones you need in the signature of the event using them. As an example, see the code of the simple PrinterCallback.

Example:

    class PrinterCallback(TrainerCallback):
        def on_log(self, args, state, control, logs=None, **kwargs):
            _ = logs.pop("total_flos", None)
            if state.is_local_process_zero:
                print(logs)

####
on_epoch_begin

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called at the beginning of an epoch.

####
on_epoch_end

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called at the end of an epoch.

####
on_evaluate

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called after an evaluation phase.

####
on_init_end

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called at the end of the initialization of the Trainer.

####
on_log

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called after logging the last logs.

####
on_optimizer_step

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called after the optimizer step but before gradients are zeroed out. Useful for monitoring gradients.

####
on_pre_optimizer_step

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called before the optimizer step but after gradient clipping. Useful for monitoring gradients.

####
on_predict

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl metrics **kwargs )

Event called after a successful prediction.

####
on_prediction_step

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called after a prediction step.

####
on_save

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called after a checkpoint save.

####
on_step_begin

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called at the beginning of a training step. If using gradient accumulation, one training step might take several inputs.

####
on_step_end

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called at the end of a training step. If using gradient accumulation, one training step might take several inputs.

####
on_substep_end

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called at the end of an substep during gradient accumulation.

####
on_train_begin

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called at the beginning of training.

####
on_train_end

< source >

( args: TrainingArguments state: TrainerState control: TrainerControl **kwargs )

Event called at the end of training.

以下は、カスタム コールバックを PyTorch Trainer に登録する方法の例です。

    class MyCallback(TrainerCallback):
        "A callback that prints a message at the beginning of training"

        def on_train_begin(self, args, state, control, **kwargs):
            print("Starting training")

    trainer = Trainer(
        model,
        args,
        train_dataset=train_dataset,
        eval_dataset=eval_dataset,
        callbacks=[MyCallback],  # We can either pass the callback class this way or an instance of it (MyCallback())
    )

コールバックを登録する別の方法は、次のように `trainer.add_callback()` を呼び出すことです。

    trainer = Trainer(...)
    trainer.add_callback(MyCallback)
    # Alternatively, we can pass an instance of the callback class
    trainer.add_callback(MyCallback())

##
TrainerState

###
class transformers.TrainerState

< source >

( epoch: typing.Optional[float] = None global_step: int = 0 max_steps: int = 0 logging_steps: int = 500 eval_steps: int = 500 save_steps: int = 500 train_batch_size: int = None num_train_epochs: int = 0 num_input_tokens_seen: int = 0 total_flos: float = 0 log_history: typing.List[typing.Dict[str, float]] = None best_metric: typing.Optional[float] = None best_global_step: typing.Optional[int] = None best_model_checkpoint: typing.Optional[str] = None is_local_process_zero: bool = True is_world_process_zero: bool = True is_hyper_param_search: bool = False trial_name: str = None trial_params: typing.Dict[str, typing.Union[str, float, int, bool]] = None stateful_callbacks: typing.List[ForwardRef('TrainerCallback')] = None )

Parameters

- epoch (`float`, optional) — Only set during training, will represent the epoch the training is at (the decimal part being the percentage of the current epoch completed).
- global_step (`int`, optional, defaults to 0) — During training, represents the number of update steps completed.
- max_steps (`int`, optional, defaults to 0) — The number of update steps to do during the current training.
- logging_steps (`int`, optional, defaults to 500) — Log every X updates steps
- eval_steps (`int`, optional) — Run an evaluation every X steps.
- save_steps (`int`, optional, defaults to 500) — Save checkpoint every X updates steps.
- train_batch_size (`int`, optional) — The batch size for the training dataloader. Only needed when `auto_find_batch_size` has been used.
- num_input_tokens_seen (`int`, optional, defaults to 0) — When tracking the inputs tokens, the number of tokens seen during training (number of input tokens, not the number of prediction tokens).
- total_flos (`float`, optional, defaults to 0) — The total number of floating operations done by the model since the beginning of training (stored as floats to avoid overflow).
- log_history (`List[Dict[str, float]]`, optional) — The list of logs done since the beginning of training.
- best_metric (`float`, optional) — When tracking the best model, the value of the best metric encountered so far.
- best_global_step (`int`, optional) — When tracking the best model, the step at which the best metric was encountered. Used for setting `best_model_checkpoint`.
- best_model_checkpoint (`str`, optional) — When tracking the best model, the value of the name of the checkpoint for the best model encountered so far.
- is_local_process_zero (`bool`, optional, defaults to `True`) — Whether or not this process is the local (e.g., on one machine if training in a distributed fashion on several machines) main process.
- is_world_process_zero (`bool`, optional, defaults to `True`) — Whether or not this process is the global main process (when training in a distributed fashion on several machines, this is only going to be `True` for one process).
- is_hyper_param_search (`bool`, optional, defaults to `False`) — Whether we are in the process of a hyper parameter search using Trainer.hyperparameter_search. This will impact the way data will be logged in TensorBoard.
- stateful_callbacks (`List[StatefulTrainerCallback]`, optional) — Callbacks attached to the `Trainer` that should have their states be saved or restored. Relevant callbacks should implement a `state` and `from_state` function.

A class containing the Trainer inner state that will be saved along the model and optimizer when checkpointing and passed to the TrainerCallback.

In all this class, one step is to be understood as one update step. When using gradient accumulation, one update step may require several forward and backward passes: if you use `gradient_accumulation_steps=n`, then one update step requires going through n batches.

####
compute_steps

< source >

( args max_steps )

Calculates and stores the absolute value for logging, eval, and save steps based on if it was a proportion or not.

####
init_training_references

< source >

( trainer max_steps num_train_epochs trial )

Stores the initial training references needed in `self`

####
load_from_json

< source >

( json_path: str )

Create an instance from the content of `json_path`.

####
save_to_json

< source >

( json_path: str )

Save the content of this instance in JSON format inside `json_path`.

##
TrainerControl

###
class transformers.TrainerControl

< source >

( should_training_stop: bool = False should_epoch_stop: bool = False should_save: bool = False should_evaluate: bool = False should_log: bool = False )

Parameters

- should_training_stop (`bool`, optional, defaults to `False`) — Whether or not the training should be interrupted.

  If `True`, this variable will not be set back to `False`. The training will just stop.

- should_epoch_stop (`bool`, optional, defaults to `False`) — Whether or not the current epoch should be interrupted.

  If `True`, this variable will be set back to `False` at the beginning of the next epoch.

- should_save (`bool`, optional, defaults to `False`) — Whether or not the model should be saved at this step.

  If `True`, this variable will be set back to `False` at the beginning of the next step.

- should_evaluate (`bool`, optional, defaults to `False`) — Whether or not the model should be evaluated at this step.

  If `True`, this variable will be set back to `False` at the beginning of the next step.

- should_log (`bool`, optional, defaults to `False`) — Whether or not the logs should be reported at this step.

  If `True`, this variable will be set back to `False` at the beginning of the next step.

A class that handles the Trainer control flow. This class is used by the TrainerCallback to activate some switches in the training loop.

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