Fix compiler warnings in mkldnn Pooling (#56095)
Summary:
Also, add `-Werror` flag to prevent this regressions from happening in
the future
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56095
Reviewed By: walterddr
Differential Revision: D27781603
Pulled By: malfet
fbshipit-source-id: 2a404788a965c380ff9feb72d0b2d967b131371f
