[Bart] _prepare_decoder_inputs should use large negative
Also renames some things and adds a nice test.
I suspect that this didn't break integration tests because we don't have a serious integration test with decoder_input_ids set (e.g. calculating loss for a summarization example)
Thanks @tomhosking for noticing!<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3158?src=pr&el=h1) Report
> Merging [#3158](https://codecov.io/gh/huggingface/transformers/pull/3158?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6ffe03a0a1d472a4e5941793fd361d2b82c8be3f?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3158/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3158?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3158      +/-   ##
==========================================
+ Coverage   78.11%   78.12%   +<.01%     
==========================================
  Files          98       98              
  Lines       16651    16651              
==========================================
+ Hits        13007    13008       +1     
+ Misses       3644     3643       -1
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3158?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3158/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `93.57% <100%> (+0.16%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3158?src=pr&el=footer). Last update [6ffe03a...71e626b](https://codecov.io/gh/huggingface/transformers/pull/3158?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
