Separate input_ids and decoder_input_ids in model.generate()
Author: yjernite
Pull request: yes

This makes the generation behavior more similar for sequence-to-sequence and language models, and allows us to initialize decoding with a prefix for the encoder-decoder setting.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3726?src=pr&el=h1) Report
> Merging [#3726](https://codecov.io/gh/huggingface/transformers/pull/3726?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/bc65afc4dfac3badf3de3be395d4023b44c61bdd&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `89.61%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3726/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3726?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3726      +/-   ##
==========================================
+ Coverage   78.14%   78.17%   +0.03%     
==========================================
  Files         104      104              
  Lines       17723    17799      +76     
==========================================
+ Hits        13849    13915      +66     
- Misses       3874     3884      +10     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3726?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3726/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `82.77% <87.90%> (+1.29%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3726/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.81% <96.42%> (-0.17%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3726/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `68.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3726/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `97.60% <100.00%> (-0.02%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3726/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.96% <0.00%> (-0.17%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3726?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3726?src=pr&el=footer). Last update [bc65afc...6a764df](https://codecov.io/gh/huggingface/transformers/pull/3726?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Can we get the prefix functionality with just one kwarg instead of all the renaming?

Got stuck in a merge / rebase loop, closing and starting again.