[CB] Support the `num_return_sequences` argument (#42921)
* Reformat to make the code pretty
* Allow for multiple decoding sequences in CB
* Style
* Fix a generation config bug
* Add seed to example
* Batch forking
* Cahnge the fixme (for later PR)
* Copy source is optional
* Added a benchmark script for PR
* Added a test and fixed a bug
* Deepcopy and style
* Review compliance
* Style
