Add a default implementation of __torch_dispatch__
I was working on an explanation of how to call into the "super"
implementation of some given ATen operation inside of __torch_dispatch__
(https://github.com/albanD/subclass_zoo/blob/main/trivial_tensors.py)
and I kept thinking to myself "Why doesn't just calling super() on
__torch_dispatch__ work"? Well, after this patch, it does! The idea
is if you don't actually unwrap the input tensors, you can call
super().__torch_dispatch__ to get at the original behavior.
Internally, this is implemented by disabling PythonKey and then
redispatching. This implementation of disabled_torch_dispatch is
not /quite/ right, and some reasons why are commented in the code.
There is then some extra work I have to do to make sure we recognize
disabled_torch_dispatch as the "default" implementation (so we don't
start slapping PythonKey on all tensors, including base Tensors),
which is modeled the same way as how disabled_torch_function is done.
Signed-off-by: Edward Z. Yang <ezyangfb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73684
Approved by: albanD
