[T5, Testst] Add extensive hard-coded integration tests and make sure PT and TF give equal results
A direct comparison to google's official model seems quite hard - not sure if that's absolutely needed @thomwolf, @craffel

But some integration tests for T5 would be very nice, to be sure that changes in the future will not break T5.
This PR adds hard-coded integration tests, where the input for summarization is copied from Bart's summarization tests and the input for translation is taken from Appendix D of the official [paper](https://arxiv.org/pdf/1910.10683.pdf)

Checking the expected output for PT, one can see that the output looks quite good! 

- [x] Add PyTorch integration tests
- [x] Verify quality (subjectively for the moment)
- [x] Add TF integration tests
- [x] Same output for PT and TF 

UPDATE:
- Found a big bug in TF Beam Search generation (see comment) -> this PR fixes it 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3550?src=pr&el=h1) Report
> Merging [#3550](https://codecov.io/gh/huggingface/transformers/pull/3550?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6f5a12a5833d1e3783e4b8a42cb556b64085745e&el=desc) will **decrease** coverage by `0.49%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3550/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3550?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3550      +/-   ##
==========================================
- Coverage   78.30%   77.80%   -0.50%     
==========================================
  Files         100      100              
  Lines       17062    17062              
==========================================
- Hits        13360    13275      -85     
- Misses       3702     3787      +85     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3550?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/3550/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.89% <0.00%> (-27.60%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3550?src=pr&el=footer). Last update [6f5a12a...c3ce2fe](https://codecov.io/gh/huggingface/transformers/pull/3550?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
