fix bug in prepare inputs for language generation for xlm for effective batch_size > 1
if multiple sentence are to be generated the masked tokens to be appended have to equal the effective batch size
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2289?src=pr&el=h1) Report
> Merging [#2289](https://codecov.io/gh/huggingface/transformers/pull/2289?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/81db12c3ba0c2067f43c4a63edf5e45f54161042?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2289/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2289?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2289      +/-   ##
==========================================
- Coverage   73.54%   73.52%   -0.02%     
==========================================
  Files          87       87              
  Lines       14789    14792       +3     
==========================================
  Hits        10876    10876              
- Misses       3913     3916       +3
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2289?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/2289/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG0ucHk=) | `86.23% <0%> (-0.23%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2289/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `73.26% <0%> (-0.25%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2289?src=pr&el=footer). Last update [81db12c...f18ac4c](https://codecov.io/gh/huggingface/transformers/pull/2289?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Indeed, thanks @patrickvonplaten 