Add T5 Encoder for Feature Extraction
# What does this PR do?

While using T5 for feature extraction, I found out that T5 encoder provides better features than T5 decoder. Hence, it makes sense to have T5 encoder only, which should reduce the memory and inference time by half, if feature extraction is needed rather than conditional generation. 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [X] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

 T5: @patrickvonplaten
 tensorflow: @jplu

I like it! <|||||>Great, I am glad that you did like it.

Thanks @patrickvonplaten and @jplu for your feedback.

@patrickvonplaten :
I have adjusted all your code review, and also add it to the T5 documentation.
The only missing part is the tests, it will be great if you could add it.

@jplu :
I have removed the unnecessary parameters from the TF model.

Is there anything else needed from my side to merge the pull request ?<|||||>> Great, I am glad that you did like it.
> 
> Thanks @patrickvonplaten and @jplu for your feedback.
> 
> @patrickvonplaten :
> I have adjusted all your code review, and also add it to the T5 documentation.
> The only missing part is the tests, it will be great if you could add it.
> 
> @jplu :
> I have removed the unnecessary parameters from the TF model.
> 
> Is there anything else needed from my side to merge the pull request ?

I think that's great! I'll fix the tests and merge :-) <|||||>## Update:

PR is ready for review IMO. Would be great if @LysandreJik @jplu and @sgugger you can take a look :-) <|||||>> ## Update:
> PR is ready for review IMO. Would be great if @LysandreJik @jplu and @sgugger you can take a look :-)

Thanks a lot @patrickvonplaten ^_^<|||||>> Very clean implementation, thanks a lot @agemagician!

You are welcome.
I am glad that I could help making the library better, even with a small contribution.
I have to say without @patrickvonplaten help, I could not make it ^_^