message: [XLA] Rework Array constructors a bit to allow creating a Array of unique_ptr<>
  Fill() does not work with type like unique_ptr<> which cannot be copied.
  So have the basic constructor use default initialization.
diff: @@ -96,8 +96,9 @@ class Array {
  using value_type = T;
  - // Creates a new array with the specified dimensions.
  - explicit Array(absl::Span<const int64> sizes) : Array(sizes, T()) {}
  + // Creates a new array with the specified dimensions and initialized elements.
  + explicit Array(absl::Span<const int64> sizes)
  + : sizes_(sizes.begin(), sizes.end()), values_(new T[num_elements()]()) {}
  // Creates a new array with the specified dimensions and specified value for
  // every cell.
  