Emit std::string_view/std::optional from codegen (pytorch#21000)

Codegen still emitted the deprecated
`torch::executor::string_view`/`optional` aliases; emit
`std::string_view`/`std::optional` directly. Verified by building
`optimized_ops_lib` and `portable_ops_lib` locally.

Authored with Claude.

Author: cyyever

codegen/api/types/types.py

@@ -18,14 +18,14 @@
 
 halfT = BaseCppType("torch::executor", "Half")
 bfloat16T = BaseCppType("torch::executor", "BFloat16")
-stringT = BaseCppType("torch::executor", "string_view")
+stringT = BaseCppType("std", "string_view")
 scalarTypeT = BaseCppType("torch::executor", "ScalarType")
 tensorT = BaseCppType("torch::executor", "Tensor")
 tensorListT = BaseCppType("torch::executor", "TensorList")
 scalarT = BaseCppType("torch::executor", "Scalar")
 memoryFormatT = BaseCppType("torch::executor", "MemoryFormat")
 intArrayRefT = BaseCppType("torch::executor", "IntArrayRef")
-optionalT = BaseCppType("torch::executor", "optional")
+optionalT = BaseCppType("std", "optional")
 contextT = BaseCppType("torch::executor", "KernelRuntimeContext")
 
 contextExpr = Expr(