Add T5 to docs
- [x] Copy past from bart docs
- [x] polish main docs page
- [x] improve docstring in `modeling_t5.py` and `modeling_tf_t5.py`
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3461?src=pr&el=h1) Report
> Merging [#3461](https://codecov.io/gh/huggingface/transformers/pull/3461?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/3ee431dd4c720e67e35a449b453d3dc2b15ccfff&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3461/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3461?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3461   +/-   ##
=======================================
  Coverage   77.79%   77.80%           
=======================================
  Files         100      100           
  Lines       17049    17051    +2     
=======================================
+ Hits        13264    13267    +3     
+ Misses       3785     3784    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3461?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3461/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `97.58% <ø> (ø)` | |
| [src/transformers/tokenization\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3461/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdDUucHk=) | `95.83% <ø> (ø)` | |
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3461/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `81.29% <100.00%> (+0.08%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3461/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `94.98% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3461/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.32% <0.00%> (+0.17%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3461?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/3461?src=pr&el=footer). Last update [3ee431d...d57c03b](https://codecov.io/gh/huggingface/transformers/pull/3461?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
