[BREAKING CHANGE] Setting all ignored index to the PyTorch standard
Author: LysandreJik
Pull request: yes

The CrossEntropy loss, as well as other losses, accept a value as an index they will ignore when computing the loss. This value was set to -1 in some cases, but left to the default value (-100) in other cases.

To stay consistent we're setting the value to be the default PyTorch one in all cases.

Includes a few documentation fixes.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2130?src=pr&el=h1) Report
> Merging [#2130](https://codecov.io/gh/huggingface/transformers/pull/2130?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6a73382706ce3c6905023872f63a680f0eb419a4?src=pr&el=desc) will **decrease** coverage by `1.17%`.
> The diff coverage is `92.3%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2130/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2130?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2130      +/-   ##
==========================================
- Coverage   80.07%   78.89%   -1.18%     
==========================================
  Files         112      112              
  Lines       16867    16867              
==========================================
- Hits        13506    13307     -199     
- Misses       3361     3560     +199
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2130?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/2130/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RyYW5zZm9feGwucHk=) | `75.9% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/2130/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3hsbS5weQ==) | `88.34% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2130/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3JvYmVydGEucHk=) | `90.43% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_camembert.py](https://codecov.io/gh/huggingface/transformers/pull/2130/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2NhbWVtYmVydC5weQ==) | `100% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2130/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3JvYmVydGEucHk=) | `59.41% <100%> (-12.36%)` | :arrow_down: |
| [transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/2130/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2dwdDIucHk=) | `84.44% <100%> (ø)` | :arrow_up: |
| [transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2130/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2N0cmwucHk=) | `94.27% <100%> (-2.21%)` | :arrow_down: |
| [transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2130/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX29wZW5haS5weQ==) | `80.13% <100%> (-1.33%)` | :arrow_down: |
| [transformers/modeling\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/2130/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2Rpc3RpbGJlcnQucHk=) | `95.87% <100%> (ø)` | :arrow_up: |
| [transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2130/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3hsbmV0LnB5) | `72.21% <100%> (-2.33%)` | :arrow_down: |
| ... and [8 more](https://codecov.io/gh/huggingface/transformers/pull/2130/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2130?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/2130?src=pr&el=footer). Last update [6a73382...dc667ce](https://codecov.io/gh/huggingface/transformers/pull/2130?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).