[compiled autograd] support custom ops backed by c++ autograd::Function (#120681)
- Adds support for custom ops backed by c++ custom autograd functions, e.g. fbgemm
- Include files more granularly to avoid namespace pollution and circular imports
limitations:
- requires user to audit their code and opt-in their custom autograd::Function via autograd::Function::is_traceable and maybe additional compiled_args + apply_with_saved implementation. this was the only way I can think of for soundness
- will throw if we can't hash the saved_data i.e. for any non implemented type other than list and dict in at::IValue::hash https://github.com/pytorch/pytorch/blob/b0cfa96e82d7fbd02f5dbcef2632714caf89615d/aten/src/ATen/core/ivalue.cpp#L364
- can technically silently fail if both the typeid hash and the typeid string name of the custom autograd::Function collide at the same time, and an identical autograd graph containing a different custom autograd::Function, yet that has an identical implementation, is called. this case seems extremely unlikely, and the only alternative to hash collision i can think of is compiling with reflection
- tensors not saved via save_variables are not lifted, and are specialized on TensorImpl*'s hash (treated as a memory address). if needed, we can lift them.
Differential Revision: [D54818488](https://our.internmc.facebook.com/intern/diff/D54818488)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120681
Approved by: https://github.com/jansel
