Fix clamp fusion on missing limits (#17533)
Summary:
Fixes #17449
Context: before #17186, we don't fuse `clamp` for the case when `min/max` are missing inputs, because they are `prim::None` node, after #17186, we make None a `prim::Constant` node which enables the fusion for `clamp`. But codegen.cpp does not handle the case when `prim::Constant` is not a Double/Int/Bool, this PR makes it so that missing inputs are handled correctly, it is done in the following way:
1. emit nothing when you see `type? = prim::Constant()`
2. when emitRHS, do special casing for aten::clamp
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17533
Differential Revision: D14238450
Pulled By: wanchaol
fbshipit-source-id: 61a272154754b13e89021bb86002927f02cde19c
