Replace c10:: types with std and drop redundant optional boxing in tests

[cyyever]

Use std::optional/std::string_view instead of c10:: in tests, and remove redundant std::optional boxing (plain types + implicit conversion where the target is already optional).

cc @SS-JIA @manuelcandales @digantdesai @cbilgin @larryliu0820 @mergennachin @cccclai @helunwencser @jackzhxng

[pytorch-bot]

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[cyyever]

@pytorchbot label "release notes: none"

[cyyever]

@pytorchbot label "release notes: none"