[T5, examples] replace heavy t5 models with tiny random models
As first done by @sshleifer in #3488 , this PR puts a tiny T5 model on S3 to save testing time.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3556?src=pr&el=h1) Report
> Merging [#3556](https://codecov.io/gh/huggingface/transformers/pull/3556?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ae6834e028ecdf7fdbe886c1f86d0e02d5fef6f0&el=desc) will **not change** coverage by `%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3556/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3556?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3556   +/-   ##
=======================================
  Coverage   77.80%   77.80%           
=======================================
  Files         100      100           
  Lines       17064    17064           
=======================================
  Hits        13277    13277           
  Misses       3787     3787           
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3556?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/3556?src=pr&el=footer). Last update [ae6834e...2e14442](https://codecov.io/gh/huggingface/transformers/pull/3556?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
