Reland "Simple Custom Operator API, V0 (#98440)" (#99416)
See the original PR (#98440) for the description. It broke internal
builds due to proxy_tensor.py not importing torch._dynamo, which is
being fixed in the previous PR in the stack.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99416
Approved by: https://github.com/soulitzer, https://github.com/bdhirsh
