Fix RoBERTa/XLNet Pad Token in run_multiple_choice.py
`convert_examples_to_fes atures` sets `pad_token=0` by default, which is correct for BERT but incorrect for RoBERTa (`pad_token=1`) and XLNet (`pad_token=5`). I think the other arguments to `convert_examples_to_features` are correct, but it might be helpful if someone checked who is more familiar with this part of the codebase.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3631?src=pr&el=h1) Report
> Merging [#3631](https://codecov.io/gh/huggingface/transformers/pull/3631?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/243e687be6cd701722cce050005a2181e78a08a8&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3631/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3631?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3631      +/-   ##
==========================================
- Coverage   78.30%   78.28%   -0.02%     
==========================================
  Files         104      104              
  Lines       17627    17627              
==========================================
- Hits        13802    13800       -2     
- Misses       3825     3827       +2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3631?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3631/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `93.45% <0.00%> (-0.17%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3631/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.97% <0.00%> (-0.13%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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