Changes to NER examples for PLT and TPU
Author: srush
Pull request: yes

* Simplify the NER example to support new features added for us by pytorch-lightning. 
* Pull out all rank and backend special casing in the code base.  
* Setup data so that TPU examples work using the new code base. 

Testing: 
* Verify that standard examples of training work. 
* Confirm that new TPU code works and runs https://colab.research.google.com/drive/1dBN-wwYUngLYVt985wGs_OKPlK_ANB9D

In my example, I see a 4x speedup over colab GPU and multi-gpu k40, but a slow down on loading and saving model. So certainly a win for larger datsets.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3053?src=pr&el=h1) Report
> Merging [#3053](https://codecov.io/gh/huggingface/transformers/pull/3053?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8bcb37bfb80d77e06001f989ad982c9961a69c31?src=pr&el=desc) will **decrease** coverage by `1.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3053/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3053?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3053      +/-   ##
==========================================
- Coverage    77.2%   76.16%   -1.04%     
==========================================
  Files          98       98              
  Lines       16063    16063              
==========================================
- Hits        12401    12234     -167     
- Misses       3662     3829     +167
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3053?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3053/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `8.72% <0%> (-81.21%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/3053/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `85.71% <0%> (-10%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/3053/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `73.48% <0%> (-2.3%)` | :arrow_down: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/3053/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `96.03% <0%> (-2.21%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/3053/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.2% <0%> (-1.35%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3053/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `92.22% <0%> (-0.33%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3053?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/3053?src=pr&el=footer). Last update [8bcb37b...9dc0964](https://codecov.io/gh/huggingface/transformers/pull/3053?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


@LysandreJik This doesn't touch the main code, so should be fine to merge.