Make CI error on inductor fallback when decomp is available (#99473)
Fixes #99446
Remove the warning, as that annoyed end-users who don't know what to do about it.
Instead, try to hold the line by preventing any decomp from being added without making
the corresponding change to inductor's fallbacks.
Note: we probably still need to better document how to update inductor's decomps,
for now it's pretty much "go ask the inductor team for advice"
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99473
Approved by: https://github.com/ezyang, https://github.com/ngimel, https://github.com/jansel
