Changed behavior of __array_namespace__

usm_ndarray constructor's array_namespace argument's default value of None
is now interpreted to mean that usm_ndarray.__array_namespace__ returns
dpctl.tensor, rather than None.

This directly affects how Python operator special methods of usm_ndarray
behave. They would now look up corresponding attribute in dpctl.tensor
namespace.

Author: oleksandr-pavlyk

dpctl/tensor/_usmarray.pyx

@@ -884,7 +884,7 @@ cdef class usm_ndarray:
         Returns array namespace, member functions of which
         implement data API.
         """
-        return self.array_namespace_
+        return self.array_namespace_ if self.array_namespace_ is not None else dpctl.tensor
 
     def __bool__(self):
         if self.size == 1: