c10::nullopt -> std::nullopt

Summary:
X-link: pytorch/audio#3848

X-link: pytorch/ao#1151

X-link: facebookincubator/AITemplate#1032

Reviewed By: houseroad

Differential Revision: D64835967

Author: r-barnes

runtime/core/exec_aten/exec_aten.h

@@ -61,8 +61,8 @@ template <typename T>
 using ArrayRef = c10::ArrayRef<T>;
 template <typename T>
 using optional = std::optional<T>;
-using nullopt_t = c10::nullopt_t;
-using c10::nullopt;
+using nullopt_t = std::nullopt_t;
+using std::nullopt;
 using ScalarType = at::ScalarType;
 using Scalar = c10::Scalar;
 using MemoryFormat = c10::MemoryFormat;