corrected documentation for past tensor shape for ctrl and gpt2 model
fix issue #1904
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2077?src=pr&el=h1) Report
> Merging [#2077](https://codecov.io/gh/huggingface/transformers/pull/2077?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/35ff345fc9df9e777b27903f11fa213e4052595b?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2077/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2077?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2077   +/-   ##
=======================================
  Coverage   83.16%   83.16%           
=======================================
  Files         109      109           
  Lines       15858    15858           
=======================================
  Hits        13188    13188           
  Misses       2670     2670
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2077?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2077/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2N0cmwucHk=) | `96.47% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_tf\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2077/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX2N0cmwucHk=) | `97.86% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/2077/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2dwdDIucHk=) | `84.44% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/2077/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX2dwdDIucHk=) | `94.75% <ø> (ø)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2077?src=pr&el=footer). Last update [35ff345...d0383e4](https://codecov.io/gh/huggingface/transformers/pull/2077?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM, merging!