Kill LegacyBridge functions that don't do multiple dispatch. (#18696)
Summary:
At some point, we needed these functions to deal with autograd dispatching to the sparse of TH version of a backwards. But we rewrote all backwards definitions in terms of native functions, so this is no longer necessary.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18696
Differential Revision: D14710834
Pulled By: gchanan
fbshipit-source-id: b22568c58eefc79d672555bd8832398ccd965cb7
