Fix attn mask gpt2 when using past
Author: patrickvonplaten
Pull request: yes

- fixed issue #3031 
- updated doc-string for GPT2
- added two test for GPT2 that I think are important: 
        - check that when using past as an input to speed up decoding, the results are equivalent to not using past.
       - check that when using past and attn_mask as an input to speed up decoding, the results are equivalent to not using past where as an input_id slice was corrupted that is masked by the attn_mask. 

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3033?src=pr&el=h1) Report
> Merging [#3033](https://codecov.io/gh/huggingface/transformers/pull/3033?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/bb7c46852051f7d031dd4be0240c9c9db82f6ed9?src=pr&el=desc) will **decrease** coverage by `1.03%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3033/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3033?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3033      +/-   ##
==========================================
- Coverage   77.26%   76.23%   -1.04%     
==========================================
  Files          98       98              
  Lines       16047    16048       +1     
==========================================
- Hits        12399    12234     -165     
- Misses       3648     3814     +166
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3033?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/3033/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `86.16% <100%> (+0.04%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3033/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `8.72% <0%> (-81.21%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/3033/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `85.71% <0%> (-10%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/3033/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `73.48% <0%> (-2.3%)` | :arrow_down: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/3033/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `96.03% <0%> (-2.21%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/3033/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.2% <0%> (-1.35%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3033/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `92.22% <0%> (-0.17%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3033?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3033?src=pr&el=footer). Last update [bb7c468...0909d8e](https://codecov.io/gh/huggingface/transformers/pull/3033?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).