Fixing dispatch error in windows debug builds (#24360)
Summary:
nullptr initialization values for dispatch pointers were overwriting values set using the REGISTER_DISPATCH macro.
Relevant issue: https://github.com/pytorch/pytorch/issues/22681
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24360
Differential Revision: D17952241
Pulled By: ezyang
fbshipit-source-id: 4bf86dc24153e504bbeacb526c58fd8230bb972a
