Implement initial version of autograd with named tensors (#25604)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25604
In this initial version:
- autograd ignores all names.
- tensor.grad is unnamed, unless the user manually assigns to it.
- if a grad tensor has any names, perhaps the user was hoping for some
alignment-checking behavior that named tensor offers for other ops. We
raise a warning in this case.
Future: do some more extensive checking to see if this actually works in
all cases.
Test Plan:
- [namedtensor ci]
- Check a warning is raised if a grad tensor has names.
- Check tensor.grad field is unnamed.
- Check that we can perform backward on an op that doesn't explictly
support names in backward. `sigmoid` is one such op.
Differential Revision: D17171788
Pulled By: zou3519
fbshipit-source-id: 64837fde94d8269610b6d3539ac025516dbe1df4
