support python mlx.array creation from list of mlx.array's (#325)

* support python mlx.array creation from list of mlx.array's

* include bfloat16 in UT

* refactor so that sub array made of all python primitive types gets initialized by fill_vector

* address PR comment: arr.shape().size() -> arr.ndim()

* address PR comment: get back Dtype constness and let stack to handle type promotions automatically

mlx/ops.cpp

@@ -714,6 +714,7 @@ array stack(
   }
   return concatenate(new_arrays, axis, s);
 }
+
 array stack(const std::vector<array>& arrays, StreamOrDevice s /* = {} */) {
   return stack(arrays, 0, s);
 }