[Pipeline, Generation]  tf generation pipeline bug
Generation can currently not be used for TF due to a small bug. This PR fixes it and adds tests.

I will update the usage docs and pipeline notebook tomorrow.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4217?src=pr&el=h1) Report
> Merging [#4217](https://codecov.io/gh/huggingface/transformers/pull/4217?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8bf7312654d40cea1a399e86f9fe8e39e1ea3a1e&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/4217/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4217?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4217   +/-   ##
=======================================
  Coverage   78.38%   78.39%           
=======================================
  Files         120      120           
  Lines       19916    19918    +2     
=======================================
+ Hits        15611    15614    +3     
+ Misses       4305     4304    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4217?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/4217/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `76.36% <100.00%> (+0.09%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4217/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.96% <0.00%> (-0.13%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4217/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.93% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4217/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `74.38% <0.00%> (+0.41%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4217?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/4217?src=pr&el=footer). Last update [8bf7312...0e6bf4e](https://codecov.io/gh/huggingface/transformers/pull/4217?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>^^ I borked your PR unfortunately, @patrickvonplaten  (me = 💩). Sorry about that. Care to open a new one?<|||||>(or commit your change directly as others have approved)<|||||>No worries, I used my bullet proof manual rebase of copy-paste files :D. 

This looks good to me. Can you check quickly and merge? @julien-c 