Make needs_dynamic_casting multiple-complex-type aware. (#39255)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39255
We don't actually cast between these complex representations, but the prior implementation would indicate that we needed to dynamic_cast,
because we didn't have mappings for std::complex or thrust::complex.
This PR makes it so they all map to the same dtype.
Note that this has no functional change as all the use sites have already been changed to take this into account.
Test Plan: Imported from OSS
Differential Revision: D21789694
Pulled By: gchanan
fbshipit-source-id: 6127aab32c40e62bf1b60fe5ccaeffacc60e3b52
