Change masking to direct labeling for TPU support.
Torch XLA conversion is very sensitive to certain core pytorch operations. These result in TPU being slower than CPU operation. The most obvious are binary masking and calls to item(). 

https://github.com/pytorch/xla/blob/master/TROUBLESHOOTING.md

This PR replaces some of these calls that are in the example/NER pathway. 

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2982?src=pr&el=h1) Report
> Merging [#2982](https://codecov.io/gh/huggingface/transformers/pull/2982?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d1ab1fab1be7199e082129dfbe46eb52bca92799?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `62.5%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2982/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2982?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2982      +/-   ##
==========================================
+ Coverage   74.09%   74.09%   +<.01%     
==========================================
  Files          93       93              
  Lines       15249    15253       +4     
==========================================
+ Hits        11298    11301       +3     
- Misses       3951     3952       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2982?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2982/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `64.95% <0%> (-0.31%)` | :arrow_down: |
| [src/transformers/modeling\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/2982/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19kaXN0aWxiZXJ0LnB5) | `95.87% <100%> (+0.01%)` | :arrow_up: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2982/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `87.92% <100%> (+0.02%)` | :arrow_up: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2982/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `73.25% <50%> (+0.04%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2982?src=pr&el=footer). Last update [d1ab1fa...757e2c3](https://codecov.io/gh/huggingface/transformers/pull/2982?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
