add generate method to SpeechT5ForTextToSpeech
Author: ylacombe
Pull request: yes

# What does this PR do?

This simple PR aims at adding a `generate` method to `SpeechT5ForTextToSpeech`, which does exactly the same than `generate_speech`. 

`generate_speech` was left for backward compatibility.

The goal is to make `SpeechT5ForTextToSpeech` compatible with the [incoming TTS pipeline](https://github.com/huggingface/transformers/pull/24952) which should not implement any special cases for older models. More on the matter in [this comment](https://github.com/huggingface/transformers/pull/24952#pullrequestreview-1556507240).



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- [x] Did you make sure to update the documentation with your changes?
- [x] Did you write any new necessary tests?


## Who can review?

@sanchit-gandhi and @sgugger , WDYT?
Thanks for your help!


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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @sgugger

Integrations:

- deepspeed: HF Trainer/Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam

Documentation: @sgugger, @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: @sgugger
- TensorFlow: @Rocketknight1

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Comments:
cc @gante as well

_The documentation is not available anymore as the PR was closed or merged._

Hi @sanchit-gandhi and @sgugger , thanks for the review! 

I would like to add `SpeechT5ForTextToSpeechWithHiFiGAN` in another PR if that's ok with you, since it requires additional tests, and since the changes made in the current PR are enough to use `SpeechT5ForTextToSpeech` with the incoming TTS pipeline!

I can open an issue to talk about `SpeechT5ForTextToSpeechWithHiFiGAN` in the meantime if you want,

thanks 

Yep good with me to add in a follow-up PR!