[LTC] Code-gen native_layer_norm_backward
Summary:
This commit code-generated native_layer_norm_backward, and improved the code-gen engine a bit on how to convert VectorCType to std::vector<>.
Test Plan:
lazy_tensor_core/test/cpp/build/test_ptltc --gtest_filter=AtenLtcTsTensorTest.TestLayerNormBackward
