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"
)
```

.github/workflows/self-scheduled.yml

@@ -38,6 +38,7 @@ jobs:
           pip install .[integrations,sklearn,testing,onnxruntime,sentencepiece,torch-speech,vision,timm]
           pip install https://github.com/kpu/kenlm/archive/master.zip
           python -m pip install 'git+https://github.com/facebookresearch/detectron2.git'
+          wandb login ${{ secrets.WANDB_API_KEY }}
 
       - name: Are GPUs recognized by our DL frameworks
         run: |
@@ -271,6 +272,7 @@ jobs:
           pip install .[integrations,sklearn,testing,onnxruntime,sentencepiece,torch-speech,vision,timm]
           pip install https://github.com/kpu/kenlm/archive/master.zip
           python -m pip install 'git+https://github.com/facebookresearch/detectron2.git'
+          wandb login ${{ secrets.WANDB_API_KEY }}
 
       - name: Are GPUs recognized by our DL frameworks
         run: |