Serializing XLMRobertaTokenizer
I am currently trying to use the XLMRobertaTokenizer in a multiprocessor setting. To do this, the XLMRobertaTokenizer needs to be serializable. Currently XLMRobertaTokenizer is not serializable while other tokenizers such as AlbertTokenizer are.

This PR adds the __getstate__ and __setstate__ methods to XLMRobertaTokenizer so that it can be serialized.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2414?src=pr&el=h1) Report
> Merging [#2414](https://codecov.io/gh/huggingface/transformers/pull/2414?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0ffc8eaf53542092271a208a52e881668e753e72?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `16.66%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2414/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2414?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #2414      +/-   ##
=========================================
- Coverage   73.24%   73.2%   -0.05%     
=========================================
  Files          87      87              
  Lines       14989   15000      +11     
=========================================
+ Hits        10979   10980       +1     
- Misses       4010    4020      +10
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2414?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2414/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `32.91% <16.66%> (-3.86%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2414?src=pr&el=footer). Last update [0ffc8ea...1d332a7](https://codecov.io/gh/huggingface/transformers/pull/2414?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Is anything blocking from merging this? :)
Would help us a lot with parallelizing the preprocessing!<|||||>Awesome! Thanks @LysandreJik  :)