[LTC] Code-gen layer_norm
Summary:
This pull request code-generated layer_norm. In order to do that, it also i) introduced fixes for optional Tensor inputs, and ii) mandated parameters being populated in the same order defined in the native function schema.
Test Plan:
lazy_tensor_core/test/cpp/build/test_ptltc --gtest_filter=AtenLtcTsTensorTest.TestLayerNorm
