Unify label args
Following up from #4711, this is a proposal to deprecate any argument that's not `labels` (like `madked_lm_labels`, `lm_labels`, etc.) to `labels`. 

I've only done one model for now to get feedback on the design, once we have something you like, I can do them all (or have separate PRs if you think that's best). 
Like it!<|||||>Added a tentative documentation for the kwargs, not sure if we want it or not.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4722?src=pr&el=h1) Report
> Merging [#4722](https://codecov.io/gh/huggingface/transformers/pull/4722?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/47a551d17b6ed2eaf03301f049006d559fca5cf3&el=desc) will **increase** coverage by `0.05%`.
> The diff coverage is `95.34%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4722/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4722?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4722      +/-   ##
==========================================
+ Coverage   77.14%   77.19%   +0.05%     
==========================================
  Files         128      128              
  Lines       21073    21130      +57     
==========================================
+ Hits        16256    16311      +55     
- Misses       4817     4819       +2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4722?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/4722/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `98.64% <ø> (ø)` | |
| [src/transformers/modeling\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/4722/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbmNvZGVyX2RlY29kZXIucHk=) | `27.27% <ø> (ø)` | |
| [src/transformers/modeling\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/4722/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yZWZvcm1lci5weQ==) | `87.94% <ø> (ø)` | |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/4722/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `71.92% <57.14%> (-0.20%)` | :arrow_down: |
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/4722/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `83.69% <90.90%> (+0.16%)` | :arrow_up: |
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/4722/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `77.67% <100.00%> (+0.46%)` | :arrow_up: |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/4722/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `96.81% <100.00%> (+0.02%)` | :arrow_up: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/4722/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `87.17% <100.00%> (+0.21%)` | :arrow_up: |
| [src/transformers/modeling\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/4722/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19kaXN0aWxiZXJ0LnB5) | `98.18% <100.00%> (+0.02%)` | :arrow_up: |
| [src/transformers/modeling\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/4722/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbGVjdHJhLnB5) | `76.89% <100.00%> (+0.46%)` | :arrow_up: |
| ... and [4 more](https://codecov.io/gh/huggingface/transformers/pull/4722/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4722?src=pr&el=footer). Last update [47a551d...68fddbd](https://codecov.io/gh/huggingface/transformers/pull/4722?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Made the same for all models since @julien-c liked it. A few comments as I was reading and deprecating.
- I found a few more wrong docstrings (mentioning `lm_label` when the arg was called `labels`) so fixed them.
- As @patrickvonplaten mentioned on #4711, `BertForMaskedLM` should be split in two (and add a `BertWithLMHead`) to remove the `lm_labels` argument. I made this a TODO to avoid this PR become too big.
- The GPT2 and openai models also have a version with two labels (`GPT2DoubleHeadsModel` and `OpenAIDoubleHeadsModel`), I renamed `lm_labels` to `labels` there but there may still be a need for a second label. Can revert the change on those models if we want each labels arg to have a useful name.
- In `LongformerModel`, the `label` argument is not used, should it be dropped?

Also, I note that quite a few docstrings have example that don't match the model they document (for instance an electra model was using Bert as an example, but there are a few instances).