Delete nonexistent parameter from documentation fix #1115
Changed documentation of GPT2Model, GPT2LMHeadModel and GPT2DoubleHeadsModel
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1116?src=pr&el=h1) Report
> Merging [#1116](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1116?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/e08c01aa1ad63efff83548ea69d5ba3ce4a75acc?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1116/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1116?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1116   +/-   ##
=======================================
  Coverage   79.61%   79.61%           
=======================================
  Files          42       42           
  Lines        6898     6898           
=======================================
  Hits         5492     5492           
  Misses       1406     1406
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1116?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1116/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfZ3B0Mi5weQ==) | `75.84% <ø> (ø)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1116?src=pr&el=footer). Last update [e08c01a...c8933bb](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1116?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks @CrafterKolyan!