Generalize Python dispatcher testing API; disallow overwriting fallback (#40469)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40469
- The old testing interface C._dispatch_import was based off the old
c10::import variation, which meant the API lined up in a strange
way with the actual torch/library.h. This diff reduces the
differences by letting you program the Library constructor directly.
- Using this newfound flexibility, we add a test for backend fallbacks
from Python; specifically testing that we disallow registering a
backend fallback twice.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Differential Revision: D22236351
Pulled By: ezyang
fbshipit-source-id: f8365e3033e9410c7e6eaf9f78aa32e1f7d55833
