[TFT5, Cache] Add cache to TFT5
This PR adds caching for TF T5.

This PR is a straight-forward translation from the caching mechanism introduced in PR: #3682.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3772?src=pr&el=h1) Report
> Merging [#3772](https://codecov.io/gh/huggingface/transformers/pull/3772?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7972a4019f4bc9f85fd358f42249b90f9cd27c68&el=desc) will **increase** coverage by `0.07%`.
> The diff coverage is `90.73%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3772/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3772?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3772      +/-   ##
==========================================
+ Coverage   78.26%   78.33%   +0.07%     
==========================================
  Files         106      106              
  Lines       17928    18027      +99     
==========================================
+ Hits        14031    14122      +91     
- Misses       3897     3905       +8     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3772?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/3772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `97.03% <80.00%> (-1.22%)` | :arrow_down: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/3772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `85.93% <86.66%> (-0.28%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/3772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `95.43% <86.66%> (-0.75%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/3772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9jdHJsLnB5) | `98.40% <87.50%> (-1.18%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.96% <90.90%> (-0.13%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `95.16% <91.59%> (+0.17%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.88% <92.30%> (+0.08%)` | :arrow_up: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `97.01% <100.00%> (ø)` | |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `96.48% <100.00%> (-0.02%)` | :arrow_down: |
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3772/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `83.80% <100.00%> (+0.59%)` | :arrow_up: |
| ... and [3 more](https://codecov.io/gh/huggingface/transformers/pull/3772/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3772?src=pr&el=footer). Last update [7972a40...d9c7a86](https://codecov.io/gh/huggingface/transformers/pull/3772?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
