Support Span to construct from a single element similar to ArrayRef

Pull Request resolved: #11757

Copy the constructor from ArrayRef here
https://github.com/pytorch/executorch/blob/5365c5559accc7a0d522eacdfb7385ce8914ef53/runtime/core/array_ref.h#L78-L81

so it's easier to use
ghstack-source-id: 291106564
@exported-using-ghexport

Differential Revision: [D76825662](https://our.internmc.facebook.com/intern/diff/D76825662/)

Author: cccclai

runtime/core/span.h

@@ -55,6 +55,10 @@ class Span final {
   template <size_t N>
   /* implicit */ constexpr Span(T (&Arr)[N]) : data_(Arr), length_(N) {}
 
+  /// Construct a Span from a single element reference.
+  /* implicit */ constexpr Span(T& single_element)
+      : data_(&single_element), length_(1) {}
+
   /// @returns a pointer to the start of the underlying element buffer.
   iterator begin() const noexcept {
     return data_;

runtime/core/test/span_test.cpp

@@ -61,3 +61,19 @@ TEST(SpanTest, TriviallyCopyable) {
   EXPECT_EQ(span.size(), span_copy.size());
   EXPECT_TRUE(std::is_trivially_copyable<Span<int64_t>>::value);
 }
+
+TEST(SpanTest, SingleElementConstructor) {
+  int64_t single_value = 42;
+  Span<int64_t> span = single_value;
+
+  EXPECT_EQ(span.size(), 1);
+  EXPECT_EQ(span.data(), &single_value);
+  EXPECT_EQ(span[0], 42);
+  EXPECT_EQ(*span.begin(), 42);
+  EXPECT_EQ(span.end(), span.begin() + 1);
+
+  // Test that modifying through span affects original value
+  span[0] = 100;
+  EXPECT_EQ(single_value, 100);
+  EXPECT_EQ(span[0], 100);
+}