Add t5 to pipeline(task='summarization')
This PR:

- adds T5 to summarization piplines.
- adds warnings and better defaults to Bart/T5 summarization
- removes unnecessary assert in generate() function 


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3413?src=pr&el=h1) Report
> Merging [#3413](https://codecov.io/gh/huggingface/transformers/pull/3413?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e392ba6938f50655a195ea7ec8a260b1e9fc6058&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `93.75%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3413/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3413?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3413      +/-   ##
==========================================
+ Coverage   77.56%   77.58%   +0.02%     
==========================================
  Files         100      100              
  Lines       16970    16993      +23     
==========================================
+ Hits        13162    13184      +22     
- Misses       3808     3809       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3413?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3413/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.71% <ø> (-0.02%)` | :arrow_down: |
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/3413/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `73.05% <93.10%> (+0.52%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3413/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `84.44% <100.00%> (+0.52%)` | :arrow_up: |
| [src/transformers/tokenization\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3413/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdDUucHk=) | `95.89% <100.00%> (+0.05%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3413?src=pr&el=footer). Last update [e392ba6...23778d1](https://codecov.io/gh/huggingface/transformers/pull/3413?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
