Trainer with grad accum
Author: sgugger
Pull request: yes

As mentioned on the forum, the behavior of `Trainer` can be confusing when using gradient accumulation as the count of steps becomes the count of update steps, not the count of training examples seen. This PR adds a warning in the doc.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6930?src=pr&el=h1) Report
> Merging [#6930](https://codecov.io/gh/huggingface/transformers/pull/6930?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/207ed8cb78ceb4980e40c89f867b06202e660395?el=desc) will **decrease** coverage by `3.53%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6930/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6930?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6930      +/-   ##
==========================================
- Coverage   80.60%   77.07%   -3.54%     
==========================================
  Files         161      161              
  Lines       30119    30119              
==========================================
- Hits        24278    23214    -1064     
- Misses       5841     6905    +1064     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6930?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/training\_args.py](https://codecov.io/gh/huggingface/transformers/pull/6930/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `91.66% <ø> (ø)` | |
| [src/transformers/training\_args\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/6930/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzX3RmLnB5) | `47.45% <ø> (ø)` | |
| [src/transformers/configuration\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/6930/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2x4bWVydC5weQ==) | `20.00% <0.00%> (-80.00%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/6930/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9seG1lcnQucHk=) | `22.49% <0.00%> (-71.63%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/6930/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hbGJlcnQucHk=) | `21.47% <0.00%> (-69.44%)` | :arrow_down: |
| [src/transformers/modeling\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/6930/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19seG1lcnQucHk=) | `23.50% <0.00%> (-67.27%)` | :arrow_down: |
| [src/transformers/tokenization\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/6930/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYWxiZXJ0LnB5) | `28.84% <0.00%> (-58.66%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/6930/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `60.81% <0.00%> (-22.62%)` | :arrow_down: |
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6930/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `20.53% <0.00%> (-21.21%)` | :arrow_down: |
| [src/transformers/modeling\_transfo\_xl\_utilities.py](https://codecov.io/gh/huggingface/transformers/pull/6930/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190cmFuc2ZvX3hsX3V0aWxpdGllcy5weQ==) | `52.98% <0.00%> (-13.44%)` | :arrow_down: |
| ... and [15 more](https://codecov.io/gh/huggingface/transformers/pull/6930/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6930?src=pr&el=footer). Last update [207ed8c...68c12f3](https://codecov.io/gh/huggingface/transformers/pull/6930?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).