(#16825)

(#16825)

Summary:
setting the correct math type for cudnn rnn, which is enforced starting from cudnn 7.5+

1. Updating persistent rnn check with input data type instead of rnn math type;
2. Updating rnn type promotion to set correct math type for accumulation;
3. Replace datatype check for filter descriptor from rnn.datatype to input.datatype;
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16825

Differential Revision: D14071190

Pulled By: ezyang

fbshipit-source-id: 1c9a1531ccf510cb0619e830be444c20c5e72f3f