Add _assert_scalar and teach Inductor to codegen it (#114148)
Inductor codegen for `_assert_async` is currently disabled because we don't really understand how to codegen `scalar_to_tensor` on a Sympy expression. I initially tried to see if I could get this to work, but I got into some weird problem involving stride sorting, so I decided to fix it properly by not going through a tensor.
So we introduce an `_assert_scalar` which takes a scalar as an argument, avoiding needing to turn a SymBool into a tensor before asserting on it. I also add `_functional_assert_scalar` for good luck, although this doesn't do anything right now because https://github.com/pytorch/pytorch/pull/104203 still hasn't been landed.
I need to customize the codegen for this operator, so I decide to directly implement it in Inductor, rather than trying to treat it as a generic ExternKernel. This leads to the new AssertScalar IR node. This is written carefully so that it doesn't get DCE'd by Inductor.
Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/114148
Approved by: https://github.com/jansel
ghstack dependencies: #120800
