Fix GPT2LMHeadModel.from_pretrained(from_tf=True)
GPT2LMHeadModel.from_pretrained(from_tf=True) doesn't work because pointer points to the GPT2LMHeadModel instance, not the GPT2Model instance. 

This bug causes errors like:

'GPT2LMHeadModel' object has no attribute 'h'
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1796?src=pr&el=h1) Report
> Merging [#1796](https://codecov.io/gh/huggingface/transformers/pull/1796?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b5d330d11820f4ac2cc8c909b1a6a77e0cd961e0?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1796/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1796?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1796      +/-   ##
==========================================
- Coverage   84.03%   84.02%   -0.02%     
==========================================
  Files          94       94              
  Lines       14032    14034       +2     
==========================================
  Hits        11792    11792              
- Misses       2240     2242       +2
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1796?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/1796/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2dwdDIucHk=) | `83.91% <0%> (-0.54%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/1796?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/1796?src=pr&el=footer). Last update [b5d330d...60ebaa5](https://codecov.io/gh/huggingface/transformers/pull/1796?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Looks good to me!<|||||>Are you loading from a TF 1.0 or a TF 2.0 model?<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
