[CUDNN] Update tests and dispatching for CUDNN V8 API behavior for `bfloat16` convs (#81139)
cuDNN via the V8 API supports `bfloat16` on Ampere (`>= (8, 0)` but not older devices) which might be unexpected given current test settings. This PR fixes some dispatching to check the device capability before dispatching `bfloat16` convs and adjusts the expected failure conditions for the autocast test.
CC @xwang233 @ptrblck
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81139
Approved by: https://github.com/ngimel
