Extend impl_backward to be usable with torch.library operators (#106817)
- impl_save_for_backward/impl_backward only work for functional,
non-view schemas. We validate this.
- impl_save_for_backward/impl_backward raise if there already exists an
autograd implementation from torch.library / TORCH_LIBRARY.
- Operators constructed via custom_op receive an "autograd indirection
kernel". The "autograd indirection kernel" automatically pulls the
constructed autograd kernel out of a dict. When
impl_save_for_backward/impl_backward get used with torch.library
operators, we also register the "autograd indirection kernel" so we can
reuse the logic.
Test Plan:
- new tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106817
Approved by: https://github.com/soulitzer
ghstack dependencies: #106799, #106800
