docs(wandb): explain how to use W&B integration
Documentation on how to use W&B integration has been added to clear up confusion on how to customize logging.

fix #5262
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5607?src=pr&el=h1) Report
> Merging [#5607](https://codecov.io/gh/huggingface/transformers/pull/5607?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/40d98ebf50c4662bcd6dce6395bbed0b2142ea52&el=desc) will **increase** coverage by `1.23%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5607/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5607?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5607      +/-   ##
==========================================
+ Coverage   76.88%   78.11%   +1.23%     
==========================================
  Files         145      145              
  Lines       25351    25351              
==========================================
+ Hits        19491    19804     +313     
+ Misses       5860     5547     -313     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5607?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5607/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `43.98% <0.00%> (-49.38%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5607/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `79.26% <0.00%> (-0.34%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5607/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `96.77% <0.00%> (+73.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5607?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5607?src=pr&el=footer). Last update [40d98eb...f38d33b](https://codecov.io/gh/huggingface/transformers/pull/5607?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
