[QOL] add signature for prepare_seq2seq_batch
Author: sshleifer
Pull request: yes

This pr adds the signature, but no implementation of `prepare_seq2seq_batch` to `PretrainedTokenizer` to allow IDE autocompletion when `AutoTokenizer` is used.
+ The signature is quite large and it is nice to see what needs to be supplied.
+ The signature is enforced by the unittests, so it won't be misleading.


On this branch, where `self.tokenizer = AutoTokenizer(...)`:
![image](https://user-images.githubusercontent.com/6045025/93033269-8ed6d480-f603-11ea-9cc4-1947d40de134.png)



Comments:
Didn't know docstring inheritance worked like that. Very cool!

Docstring looks great to me, thanks for adding!

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7108?src=pr&el=h1) Report
> Merging [#7108](https://codecov.io/gh/huggingface/transformers/pull/7108?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/206b78d4850d3c6fe85a015654293fc4b803ed7b?el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7108/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7108?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7108      +/-   ##
==========================================
+ Coverage   80.84%   80.87%   +0.02%     
==========================================
  Files         168      168              
  Lines       32284    32285       +1     
==========================================
+ Hits        26099    26109      +10     
+ Misses       6185     6176       -9     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7108?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/7108/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `100.00% <ø> (ø)` | |
| [src/transformers/tokenization\_mbart.py](https://codecov.io/gh/huggingface/transformers/pull/7108/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWJhcnQucHk=) | `96.82% <ø> (ø)` | |
| [src/transformers/tokenization\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/7108/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdDUucHk=) | `95.04% <ø> (ø)` | |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/7108/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `99.15% <100.00%> (ø)` | |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7108/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.88% <100.00%> (+0.03%)` | :arrow_up: |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/7108/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `92.83% <0.00%> (-0.36%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7108/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `83.08% <0.00%> (+0.24%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7108/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+2.25%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/7108?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/7108?src=pr&el=footer). Last update [206b78d...6adf419](https://codecov.io/gh/huggingface/transformers/pull/7108?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).