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# 使用Trainer API进行超参数搜索

🤗 Transformers库提供了一个优化过的[`Trainer`]类，用于训练🤗 Transformers模型，相比于手动编写自己的训练循环，这更容易开始训练。[`Trainer`]提供了超参数搜索的API。本文档展示了如何在示例中启用它。


## 超参数搜索后端

[`Trainer`] 目前支持四种超参数搜索后端：[optuna](https://optuna.org/)，[sigopt](https://sigopt.com/)，[raytune](https://docs.ray.io/en/latest/tune/index.html)，[wandb](https://wandb.ai/site/sweeps)

在使用它们之前，您应该先安装它们作为超参数搜索后端。

```bash
pip install optuna/sigopt/wandb/ray[tune]
```

## 如何在示例中启用超参数搜索

定义超参数搜索空间，不同的后端需要不同的格式。

对于sigopt，请参阅sigopt [object_parameter](https://docs.sigopt.com/ai-module-api-references/api_reference/objects/object_parameter)，它类似于以下内容：

```py
>>> def sigopt_hp_space(trial):
...     return [
...         {"bounds": {"min": 1e-6, "max": 1e-4}, "name": "learning_rate", "type": "double"},
...         {
...             "categorical_values": ["16", "32", "64", "128"],
...             "name": "per_device_train_batch_size",
...             "type": "categorical",
...         },
...     ]
```

对于optuna，请参阅optuna [object_parameter](https://optuna.readthedocs.io/en/stable/tutorial/10_key_features/002_configurations.html#sphx-glr-tutorial-10-key-features-002-configurations-py)，它类似于以下内容：

```py
>>> def optuna_hp_space(trial):
...     return {
...         "learning_rate": trial.suggest_float("learning_rate", 1e-6, 1e-4, log=True),
...         "per_device_train_batch_size": trial.suggest_categorical("per_device_train_batch_size", [16, 32, 64, 128]),
...     }
```

Optuna提供了多目标HPO。您可以在`hyperparameter_search`中传递`direction`参数，并定义自己的`compute_objective`以返回多个目标值。在`hyperparameter_search`中将返回Pareto Front（`List[BestRun]`），您应该参考[test_trainer](https://github.com/huggingface/transformers/blob/main/tests/trainer/test_trainer.py)中的测试用例`TrainerHyperParameterMultiObjectOptunaIntegrationTest`。它类似于以下内容：

```py
>>> best_trials = trainer.hyperparameter_search(
...     direction=["minimize", "maximize"],
...     backend="optuna",
...     hp_space=optuna_hp_space,
...     n_trials=20,
...     compute_objective=compute_objective,
... )
```

对于raytune，可以参考raytune的[object_parameter](https://docs.ray.io/en/latest/tune/api/search_space.html)，它类似于以下内容：

```py
>>> def ray_hp_space(trial):
...     return {
...         "learning_rate": tune.loguniform(1e-6, 1e-4),
...         "per_device_train_batch_size": tune.choice([16, 32, 64, 128]),
...     }
```

对于wandb，可以参考wandb的[object_parameter](https://docs.wandb.ai/guides/sweeps/configuration)，它类似于以下内容：

```py
>>> def wandb_hp_space(trial):
...     return {
...         "method": "random",
...         "metric": {"name": "objective", "goal": "minimize"},
...         "parameters": {
...             "learning_rate": {"distribution": "uniform", "min": 1e-6, "max": 1e-4},
...             "per_device_train_batch_size": {"values": [16, 32, 64, 128]},
...         },
...     }
```

定义一个`model_init`函数并将其传递给[Trainer]，作为示例：

```py
>>> def model_init(trial):
...     return AutoModelForSequenceClassification.from_pretrained(
...         model_args.model_name_or_path,
...         from_tf=bool(".ckpt" in model_args.model_name_or_path),
...         config=config,
...         cache_dir=model_args.cache_dir,
...         revision=model_args.model_revision,
...         use_auth_token=True if model_args.use_auth_token else None,
...     )
```

使用你的`model_init`函数、训练参数、训练和测试数据集以及评估函数创建一个[`Trainer`]。

```py
>>> trainer = Trainer(
...     model=None,
...     args=training_args,
...     train_dataset=small_train_dataset,
...     eval_dataset=small_eval_dataset,
...     compute_metrics=compute_metrics,
...     processing_class=tokenizer,
...     model_init=model_init,
...     data_collator=data_collator,
... )
```

调用超参数搜索，获取最佳试验参数，后端可以是`"optuna"`/`"sigopt"`/`"wandb"`/`"ray"`。方向可以是`"minimize"`或`"maximize"`，表示是否优化更大或更低的目标。

您可以定义自己的compute_objective函数，如果没有定义，将调用默认的compute_objective，并将评估指标（如f1）之和作为目标值返回。

```py
>>> best_trial = trainer.hyperparameter_search(
...     direction="maximize",
...     backend="optuna",
...     hp_space=optuna_hp_space,
...     n_trials=20,
...     compute_objective=compute_objective,
... )
```

## 针对DDP微调的超参数搜索
目前，Optuna和Sigopt已启用针对DDP的超参数搜索。只有rank-zero进程会进行超参数搜索并将参数传递给其他进程。
