[Generation Beam Search] Fix bug when changing the <EOS> token for generate
This PR fixes https://github.com/huggingface/transformers/issues/4121 . When comparing int `!=` should be used here instead of `is not`. 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4745?src=pr&el=h1) Report
> Merging [#4745](https://codecov.io/gh/huggingface/transformers/pull/4745?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/47a551d17b6ed2eaf03301f049006d559fca5cf3&el=desc) will **increase** coverage by `0.22%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4745/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4745?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4745      +/-   ##
==========================================
+ Coverage   77.14%   77.36%   +0.22%     
==========================================
  Files         128      128              
  Lines       21073    21073              
==========================================
+ Hits        16256    16304      +48     
+ Misses       4817     4769      -48     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4745?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4745/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <ø> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4745/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.29% <ø> (+0.35%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4745/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :arrow_up: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/4745/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `86.21% <0.00%> (+14.10%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4745?src=pr&el=footer). Last update [47a551d...491f4e2](https://codecov.io/gh/huggingface/transformers/pull/4745?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
