Add W&B backend for hyperparameter sweep (#14582)

# Add support for W&B hyperparameter sweep
This PR:
* allows using wandb for running hyperparameter search.
* The runs are visualized on W&B sweeps dashboard
* This supports runnning sweeps on parallel devices, all reporting to the same central dashboard.

### Usage
**To run new a hyperparameter search:**
```
trainer.hyperparameter_search(
    backend="wandb", 
    project="transformers_sweep", # name of the project
    n_trials=5,
    metric="eval/loss", # metric to be optimized, default 'eval/loss'. A warning is raised if the passed metric is not found
)
```
This outputs a sweep id. Eg. `my_project/sweep_id`

**To run sweeps on parallel devices:**
Just pass sweep id which you want to run parallel
```
trainer.hyperparameter_search(
    backend="wandb", 
    sweep_id = "my_project/sweep_id"
)
```

src/transformers/testing_utils.py

@@ -59,7 +59,7 @@ from .file_utils import (
     is_torchaudio_available,
     is_vision_available,
 )
-from .integrations import is_optuna_available, is_ray_available, is_sigopt_available
+from .integrations import is_optuna_available, is_ray_available, is_sigopt_available, is_wandb_available
 
 
 SMALL_MODEL_IDENTIFIER = "julien-c/bert-xsmall-dummy"
@@ -590,6 +590,19 @@ def require_sigopt(test_case):
         return test_case
 
 
+def require_wandb(test_case):
+    """
+    Decorator marking a test that requires wandb.
+
+    These tests are skipped when wandb isn't installed.
+
+    """
+    if not is_wandb_available():
+        return unittest.skip("test requires wandb")(test_case)
+    else:
+        return test_case
+
+
 def require_soundfile(test_case):
     """
     Decorator marking a test that requires soundfile