Avoid defining bogus CPPTypeAndStdComplexToScalarType<void> by using some decltype tricks. (#39261)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/39261
Test Plan: Imported from OSS
Differential Revision: D21790288
Pulled By: gchanan
fbshipit-source-id: c1f04577c02f78dbc911aad4cb1d862acbea4b31
