Remove unnecessary use of FusedLayerNorm
Fix #1172
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1315?src=pr&el=h1) Report
> Merging [#1315](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1315?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/a2d4950f5c909f7bb4ea7c06afa6cdecde7e8750?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1315/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1315?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1315      +/-   ##
==========================================
- Coverage   80.77%   80.76%   -0.01%     
==========================================
  Files          57       57              
  Lines        8092     8091       -1     
==========================================
- Hits         6536     6535       -1     
  Misses       1556     1556
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1315?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1315/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfYmVydC5weQ==) | `88.3% <100%> (-0.03%)` | :arrow_down: |

------

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<|||||>Note this change makes the codebase to be compatible with apex amp O1.<|||||>Ok great, thanks @bryant1410!