feat(wandb): logging and configuration improvements
# What does this PR do?

Following improvements to `wandb` integration:
* ensure unique artifact id → previously it was based on run name which could create duplicates and mismatches (as runs can be updated manually in the UI)
* allow manual calls to `wandb.init()` → previously it would have closed the run and started a new one
* when a wandb run already exists (manually created), adds automatically model config parameters
* simplify reinit logic → now explicitly closes a run for hp search and avoid use of reinit which can have complex side effects (different behavior in notebooks vs scripts)
* ensure we have no dropped values (when step is below a previous logged value) by logging the step as an independent `train/global_step` metric and set it as default x-axis in the UI (can be edited manually). Note: this auto-setting of x-axis will be activated in upcoming release of wandb
* get values committed immediately so they appear in the UI with no delay
* fixes compatibility with sagemaker

Fixes https://github.com/wandb/client/issues/1499, #8754, #10486


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [X] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

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Documentation: @sgugger