only enable cudnn persistent RNN when batchsize % 8 == 0 (#48070)
Summary:
On A100, cuDNN persistent RNN algo doesn't work quite well when batch size is not a multiple of 8, so we need to disable it.
Related: https://github.com/pytorch/pytorch/pull/43165
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48070
Reviewed By: bdhirsh
Differential Revision: D25283953
Pulled By: ngimel
fbshipit-source-id: d7f33b1f43e2e3c46dc89ae046779175f6992569
