[autocast] Clean up registrations using new macros (#86403)
This PR cleans up m.impl(...) calls to use the new KERNEL / KERNEL_CPU
macros. That saves us the trouble of writing out the signatures.
Test Plan:
- code reading
- wait for tests.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86403
Approved by: https://github.com/ezyang
