add generate method to SpeechT5ForTextToSpeech (#25233)
* add generate method to SpeechT5ForTextToSpeech
* update speecht5forTTS docstrings
* Remove defaults to None in generate docstrings
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
---------
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>