[Generate, Test] Split generate test function into beam search, no beam search
- Clean the generate testing functions 
- Also should fix flaky behaviour of bad_word_tokens test (see #3367 and https://circleci.com/gh/huggingface/transformers/27997?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link) 


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3601?src=pr&el=h1) Report
> Merging [#3601](https://codecov.io/gh/huggingface/transformers/pull/3601?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f68d22850ced09bb194b30068ff94ca3409f0879&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3601/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3601?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3601      +/-   ##
==========================================
- Coverage   78.06%   78.05%   -0.01%     
==========================================
  Files         100      100              
  Lines       17134    17134              
==========================================
- Hits        13375    13374       -1     
- Misses       3759     3760       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3601?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3601/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `93.28% <0.00%> (-0.17%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3601/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `92.10% <0.00%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3601?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/3601?src=pr&el=footer). Last update [f68d228...857e77e](https://codecov.io/gh/huggingface/transformers/pull/3601?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
