[Generate] beam search should generate without replacement
When doing beam search decoding and sampling instead of argmax (edge case, probably very rarely used), we need to sample **without** replacement. This is implemented by  default in torch, but not in TF, see https://pytorch.org/docs/master/generated/torch.multinomial.html#torch.multinomial (torch). An easy solution is to use the Gumbel max trick instead: https://github.com/tensorflow/tensorflow/issues/9260

This will fix the sometimes flaky TFBeamSearchGenerate Tests as well: #4447 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4845?src=pr&el=h1) Report
> Merging [#4845](https://codecov.io/gh/huggingface/transformers/pull/4845?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f9414f7553d3f1872b372990ef03205c0d1141df&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4845/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4845?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4845      +/-   ##
==========================================
- Coverage   76.06%   76.05%   -0.01%     
==========================================
  Files         128      128              
  Lines       21498    21502       +4     
==========================================
+ Hits        16352    16354       +2     
- Misses       5146     5148       +2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4845?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4845/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.12% <100.00%> (-0.24%)` | :arrow_down: |

------

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