fix evaluation bug
The original `run_squad.py` has a potential bug. If we only want to run the script to do evaluation, the model will not be properly loaded. The simple fix is provided.
# [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/745?src=pr&el=h1) Report
> Merging [#745](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/745?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/commit/dad3c7a485b7ffc6fd2766f349e6ee845ecc2eee?src=pr&el=desc) will **decrease** coverage by `0.05%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/745/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/745?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master     #745      +/-   ##
==========================================
- Coverage   62.27%   62.22%   -0.06%     
==========================================
  Files          18       18              
  Lines        3979     3979              
==========================================
- Hits         2478     2476       -2     
- Misses       1501     1503       +2
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/745?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_pretrained\_bert/tokenization\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/745/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uX2dwdDIucHk=) | `82.44% <0%> (-1.07%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/745?src=pr&el=footer). Last update [dad3c7a...64b2a82](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/745?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
