ByT5 model (#11971)
* allow tf to use uneven num of layers
* add tokenizer
* finish docs
* finish docs
* Apply suggestions from code review
* include in index
* finish
* Update docs/source/model_doc/byt5.rst
Co-authored-by: NielsRogge <48327001+NielsRogge@users.noreply.github.com>
* apply sylvais suggestions
* make style
Co-authored-by: NielsRogge <48327001+NielsRogge@users.noreply.github.com>
