Support Span to construct from a single element similar to ArrayRef

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

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

ghstack-source-id: 290994798
Pull Request resolved: #11757

Author: cccclai

runtime/core/span.h

@@ -55,6 +55,9 @@ 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_;