Add mkldnn conv double backward (#15686)
Summary:
Fixes #15353 .
Like cudnn conv implementation, mkldnn also falls back to the default `_convolution_double_backward` as double backward.
This bug wasn't caught by CI before because mkldnn is only used when input scalar type is float, but our tests are all using double as default.
Adding test for float inputs, but mkldnn seems to have imprecision issues similar to cudnn implementation, so here I only check if double backward exists instead of calling `gradgradcheck`. Please correct me if the precision should actually be checked.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15686
Differential Revision: D13571682
Pulled By: ailzhang
fbshipit-source-id: f1762439762370f276cfd59e8b8b8a4dee960a4b
