Fix misleading RoBERTa token type ids
RoBERTa does not actually make use of token type ids. When feeding the output of `encode_plus` used with a pair of sequences to the model directly, it crashes as it cannot handle token type ids that have a value of 1.

This fix returns a list of zeros as the token type ids instead.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2432?src=pr&el=h1) Report
> Merging [#2432](https://codecov.io/gh/huggingface/transformers/pull/2432?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9261c7f771fccfa2a2cb78ae544adef2f6eb402b?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2432/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2432?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2432   +/-   ##
=======================================
  Coverage   73.24%   73.24%           
=======================================
  Files          87       87           
  Lines       15001    15001           
=======================================
  Hits        10988    10988           
  Misses       4013     4013
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2432?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2432/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcm9iZXJ0YS5weQ==) | `100% <100%> (ø)` | :arrow_up: |

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<|||||>> 
> 
> RoBERTa does not actually make use of token type ids. When feeding the output of `encode_plus` used with a pair of sequences to the model directly, it crashes as it cannot handle token type ids that have a value of 1.
> 
> This fix returns a list of zeros as the token type ids instead.

I encountered the same problem. Thank you for your solution, I figure out what's wrong with my problem now.