[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.

PiperOrigin-RevId: 358928330
Change-Id: I76915ee0deb9c7c64bd6b8efeb6e2399c65e42e1

tensorflow/compiler/xla/array.h

@@ -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.