Expose autograd.graph.Node as an abstract base class (#91475)
This PR:
- registers all of the codegened Nodes to the torch._C._functions module, this is where special nodes like AccumulateGrad are already registered.
- creates a autograd.graph.Node abstract base class that all of the newly registered nodes subclass from. We make the subclassing happen by implementing the ``__subclasshook__`` method
- enables static type checking to work and also enables Sphinx to generate documentation for the Node and its methods
- handles both the custom Function and codegened cases
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91475
Approved by: https://github.com/albanD
