[cleanup] PretrainedModel.generate: remove unused kwargs
`_generate_beam_search` and `_generate_no_beam_search` do not use `bos_token_id` or `decoder_start_token_id`
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4761?src=pr&el=h1) Report
> Merging [#4761](https://codecov.io/gh/huggingface/transformers/pull/4761?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5bf9afbf351f9419505eb1c9e0c5ab78883c3caf&el=desc) will **decrease** coverage by `0.09%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4761/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4761?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4761      +/-   ##
==========================================
- Coverage   77.41%   77.31%   -0.10%     
==========================================
  Files         128      128              
  Lines       21059    21059              
==========================================
- Hits        16302    16282      -20     
- Misses       4757     4777      +20     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4761?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4761/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.15% <ø> (ø)` | |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/4761/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.66% <0.00%> (-6.37%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4761/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.80% <0.00%> (-0.12%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4761/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.79% <0.00%> (+0.16%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4761?src=pr&el=footer). Last update [5bf9afb...95a6ef1](https://codecov.io/gh/huggingface/transformers/pull/4761?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
