Implement usm_ndarray.__arrray__ method

The property raises TypeError informing user that
implicit conversions from usm_ndarray to numpy.ndarray
are not permitted.

Explicit conversion using `dpctl.tensor.asnumpy` is
advised.

Disallowing implicit conversion prevents `np.asarray(usm_ar)`
from creating an array of 0D usm_ndarray instances, because
using it is very costly due to multitude of short-array transfers
from GPU to host.

dpctl/tensor/_usmarray.pyx

@@ -1552,6 +1552,14 @@ cdef class usm_ndarray:
     def __repr__(self):
         return usm_ndarray_repr(self)
 
+    def __array__(self, dtype=None, copy=None):
+        "NumPy array protocol"
+        raise TypeError(
+            "Implicit conversion to a NumPy array is not allowed. "
+	    "Use `dpctl.tensor.asnumpy` to copy data from this "
+	    "`dpctl.tensor.usm_ndarray` instance to NumPy array"
+	)
+
 
 cdef usm_ndarray _real_view(usm_ndarray ary):
     """