Use native ctc loss for target length 256 (#53557)
Summary:
Apparently cudnn (8.1) does not like 256-long targets.
Thank you raotnameh for reporting.
Fixes https://github.com/pytorch/pytorch/issues/53505
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53557
Reviewed By: VitalyFedyunin
Differential Revision: D26947262
Pulled By: albanD
fbshipit-source-id: df6da7db8fd8e35050b4303ff1658646ebc60141
