[wip] T5 tokenizer should add special tokens
Thanks for taking care of it @sshleifer !<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5832?src=pr&el=h1) Report
> Merging [#5832](https://codecov.io/gh/huggingface/transformers/pull/5832?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/615be03f9d961c0c9722fe10e7830e011066772e&el=desc) will **decrease** coverage by `0.18%`.
> The diff coverage is `84.61%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5832/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5832?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5832      +/-   ##
==========================================
- Coverage   78.66%   78.48%   -0.19%     
==========================================
  Files         146      146              
  Lines       26200    26213      +13     
==========================================
- Hits        20611    20574      -37     
- Misses       5589     5639      +50     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5832?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/5832/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdDUucHk=) | `94.04% <84.61%> (-1.73%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5832/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `26.02% <0.00%> (-69.52%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5832/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.21% <0.00%> (-0.51%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5832/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.79% <0.00%> (+33.89%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5832?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5832?src=pr&el=footer). Last update [615be03...04f18d4](https://codecov.io/gh/huggingface/transformers/pull/5832?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
