seq2seq/run_eval.py can take decoder_start_token_id
Also: batch_decode: document kwargs for autocomplete
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5949?src=pr&el=h1) Report
> Merging [#5949](https://codecov.io/gh/huggingface/transformers/pull/5949?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/95d1962b9c8460b4cec5a88eb9915e8e25f5bc1e&el=desc) will **decrease** coverage by `0.17%`.
> The diff coverage is `50.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5949/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5949?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5949      +/-   ##
==========================================
- Coverage   78.69%   78.51%   -0.18%     
==========================================
  Files         146      146              
  Lines       26214    26214              
==========================================
- Hits        20628    20581      -47     
- Misses       5586     5633      +47     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5949?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/5949/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.60% <50.00%> (ø)` | |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5949/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `26.02% <0.00%> (-69.52%)` | :arrow_down: |
| [src/transformers/generation\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5949/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3V0aWxzLnB5) | `96.82% <0.00%> (-0.29%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5949/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.79% <0.00%> (+33.89%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5949?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/5949?src=pr&el=footer). Last update [95d1962...450d753](https://codecov.io/gh/huggingface/transformers/pull/5949?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
