Use std::string_view in vulkan op-test codegen

[cyyever]

torchgen emits c10::string_view for string args; normalize to std::string_view at codegen time (drop OLD_STRING). Also drop the unused c10/util/Optional.h include in exir/verification/bindings.cpp.

cc @SS-JIA @manuelcandales @digantdesai @cbilgin @larryliu0820 @JacobSzwejbka @lucylq

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