Fixes selection of cuDNN algorithm (#15881)
Summary:
This PR updates the logic for using cudnnGet* and cudnnFind*. Current version of cudnn find and get (v7) returns a pair of best algorithm and the convDesc mathType. While we were using the returned algorithm, we didn't update the mathType. As a result, we ended up with a slow choice of algorithm and math type. Without this patch, we are seeing a 10x regression in group convolutions.
Changelist:
- Changed the template arguments to be `perf_t` instead of `algo_t` to unify cudnnFind and cudnnGet. Both cudnnFind and cudnnGet have the same purpose and hence, it made sense to unify them and get rid of `getAlgorithm`.
- Used cudnnGet*_v7 everywhere cudnnGet* was being used.
- Removed all cudnn6 paths (This PR depends on https://github.com/pytorch/pytorch/pull/15851)
Differential Revision: D13957944
Pulled By: ezyang
fbshipit-source-id: a88c39d80ae37f2d686665622302b62b50fab404
