Support `text-to-speech` in `pipeline` function and in Optimum
### Feature request

SpeechT5 was recently added to Transformers:

* **Blog post**: https://huggingface.co/blog/speecht5
* **Spaces demo**: https://huggingface.co/spaces/Matthijs/speecht5-tts-demo
* **Models**: https://huggingface.co/mechanicalsea/speecht5-tts

It would be great if `text-to-speech` could be supported across the Transformers stack.

### Motivation

@xenova [bumped into this as an issue](https://github.com/xenova/transformers.js/issues/59) when trying to get SpeechT5 working in the browser (Transformers.js).

### Your contribution

Probably unable to help with this at the moment.
cc @sanchit-gandhi <|||||>Indeed, a TTS pipeline would be super helpful to run SpeechT5. We're currently planning on waiting till we have 1-2 more TTS models in the library before pushing ahead with a TTS pipeline, in order to verify that the pipeline is generalisable and gives a benefit over loading a single model + processor.

cc @hollance<|||||>Any viable contenders for the other 1-2 models? https://paperswithcode.com/task/text-to-speech-synthesis<|||||>Hey, I'd be more than happy to take up this task if we can decide on the other 1-2 models<|||||>> Hey, I'd be more than happy to take up this task if we can decide on the other 1-2 models

We can probably just select the most popular models from the hub: https://huggingface.co/models?pipeline_tag=text-to-speech&sort=downloads<|||||>There is an [open PR](https://github.com/huggingface/transformers/pull/15773) for FastSpeech2. I think this is a good new model to add. If anyone is interested in taking that PR to completion, that would be awesome!<|||||>> Hey, I'd be more than happy to take up this task if we can decide on the other 1-2 models

Let me know if you need any help! I’m excited for this to be added 🔥<|||||>Here's another model which could fall into the `text-to-speech` category: https://github.com/huggingface/transformers/issues/23036<|||||>Just added one more https://github.com/huggingface/transformers/issues/23050<|||||>Please add support for the mms-tts model as mentioned in above [issue](https://github.com/xenova/transformers.js/issues/209)  to the TTS pipeline.<|||||>Good news! This is currently being worked on: https://github.com/huggingface/transformers/pull/24952 🚀🔥