Fix fp16 masking in PoolerEndLogits
Necessary to run xlnet squad fine-tuning with `--fp16 --fp16_opt_level="O2"`, otherwise loss is immediately `NaN` and fine-tuning cannot proceed.

Similar to #1249 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1284?src=pr&el=h1) Report
> Merging [#1284](https://codecov.io/gh/huggingface/transformers/pull/1284?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/2dc8cb87341223e86220516951bb4ad84f880b4a?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `66.66%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1284/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1284?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1284      +/-   ##
==========================================
- Coverage   84.69%   84.68%   -0.01%     
==========================================
  Files          84       84              
  Lines       12596    12598       +2     
==========================================
+ Hits        10668    10669       +1     
- Misses       1928     1929       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1284?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1284/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3V0aWxzLnB5) | `92.44% <66.66%> (-0.25%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1284?src=pr&el=footer). Last update [2dc8cb8...c50783e](https://codecov.io/gh/huggingface/transformers/pull/1284?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM @slayton58 thanks