[LTC] Add `c10::optional<at::Scalar>` code-gen support (#66383)
Summary:
This commit adds code-gen support for the c10::optional<at::Scalar>
type, and adds full_codegen support for the clamp op to test the
implementation.
Test Plan:
test/cpp/build/test_ptltc --gtest_filter=AtenLtcTsTensorTest.TestClamp*
