Allow tensors with requires_grad=True in c10 ops (#21599)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21599
We prevented this because c10 ops can't have a backwards yet and calling them with requires_grad=True would do the wrong thing
if the c10 op is not purely implemented by calling other autograd-able ops.
However, it is a valid use case to have c10 ops that just call other autograd-aware ops, and these ops should be callable with requires_grad=True.
This should fix https://github.com/pytorch/pytorch/issues/21584.
Differential Revision: D15744692
fbshipit-source-id: ba665365c850ef63fc9c51498fd69afe49e5d7ec
