Upgrade run_generation
The script `run_generation` has a few issues that I aim to fix in this PR:

- [x] The XLNet and XLM generations are broken (crash)
- [x] An end of sequence token is added to all sequences, even when models don't have that token, and results in weird end of sequences.
- [x] No way to generate multiple sequences at a time as it was possible before
- [x] The `length` parameter doesn't take into account the prompt length.
- [x]  The prompt is concatenated to the generated sequence, which results in concatenating the initial text for XLNet.
- [x] Actually implement languages for XLM
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2749?src=pr&el=h1) Report
> Merging [#2749](https://codecov.io/gh/huggingface/transformers/pull/2749?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ada24def22199459d8c1decc311dfe8dae7a7d8c?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `25%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2749/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2749?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2749      +/-   ##
==========================================
- Coverage    75.1%   75.07%   -0.03%     
==========================================
  Files          93       93              
  Lines       15249    15255       +6     
==========================================
  Hits        11452    11452              
- Misses       3797     3803       +6
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2749?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2749/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `60.7% <0%> (-0.63%)` | :arrow_down: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2749/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `96.46% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2749?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/2749?src=pr&el=footer). Last update [ada24de...f2bcc91](https://codecov.io/gh/huggingface/transformers/pull/2749?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Changes have been added in PR #2885.