primTorch: support refs and decompositions when ATen and Python disagree

[nkaretnikov]

🐛 Describe the bug

1. Policy (related): We need to agree on how to handle refs/decomps like binary_cross_entropy where the ATen op and the Python frontend have different number/type of arguments. Right now, binary_cross_entropy just defines a decomp. Do we want to split the ref/decomp implementations for these (with a shared core/conversion logic) or handle in some other way?

2. Bug: if register_decomposition is used with ops like these to define one Python function that's both registered as a ref and a decomp, there are no type signature checks in register_decomposition to catch this. So it'll just run and we might not notice unless it breaks for some other reason.

I have a demo branch with this issue here, see the commit message and notes in the topmost commit:
https://github.com/nkaretnikov/pytorch/commits/primtorch-l1-loss-decomp-ref-compat-issue

Versions

master (e0f2eba)

cc @ezyang @mruberry @ngimel

[ezyang]

The aten decomp is higher priority. You can write the ref in terms of the aten decomp, but I don't think it is that important to do.