PT <-> TF for composite models (#19732)
* First step of PT->TF for composite models
* Update the tests
* For VisionEncoderDecoderModel
* Fix
* Fix
* Add comment
* Fix
* clean up import
* Save memory
* For (TF)EncoderDecoderModel
* For (TF)EncoderDecoderModel
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>
