feature: update wandb callback to upload checkpoints
# What does this PR do?

The PR updates the `WandbCallback` with the following changes:
- Adds `on_save` method to upload model checkpoints as artifacts.
- Changes the default value of environment variable `WANDB_WATCH` from `gradients` to `false`. This enables quicker training when defaults are used. The user can easily change this behavior by setting the env variable.
- Changes the `WANDB_LOG_MODEL` variable from `bool` to `str` allowing for different settings to upload artifacts.
- Modifies the class dostring to reflect the above changes.
- Fixes broken link to wandb documentation
- Changes the wandb `run_name` from `output_dir` to wandb auto generated name. this avoids duplication of run names in wandb workspace

## Before submitting

- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).


## Who can review?
- trainer: @sgugger
- documentation: @stevhliu


## Examples
- Example [colab](https://colab.research.google.com/drive/17imujjBEL2cQL3odAJEbVvjR6zVsDRuH?usp=sharing) reflecting all the changes to the WandbCallback
- Example Weights & Biases [workspace](https://wandb.ai/parambharat/hf_transformers?workspace=user-parambharat) with runs that show different settings.
- Example Weights & Biases [Artifact](https://wandb.ai/parambharat/hf_transformers/artifacts/checkpoint/checkpoint-ajdcez6k/v4) created for checkpoints
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hey, @sgugger : Thanks for the quick review and suggestions. I've resolved all the issues. :hugs:<|||||>@parambharat There is a weird issue with the tests. Could you try refreshing your permissions as shown [here](https://support.circleci.com/hc/en-us/articles/360048210711-How-to-Refresh-User-Permissions-) and then pushing an empty commit?<|||||>Hey, @sgugger. I ran the fix that you suggested and all checks pass now.<|||||>Thank you @stevhliu. I've committed your recommendations.<|||||>Thanks again for your contribution!