fix diff function (#966)

Author: Alexander-Makaryev

dpnp/dpnp_algo/dpnp_algo_mathematical.pyx

@@ -196,7 +196,7 @@ cpdef object dpnp_diff(utils.dpnp_descriptor input, int n):
             for i in range(arr.size):
                 if counter < size_idx:
                     counter += 1
-                    arr_elem = arr.item(i + 1) - arr.item(i)
+                    arr_elem = arr[numpy.unravel_index(i + 1, arr.shape)] - arr[numpy.unravel_index(i, arr.shape)]
                     res.append(arr_elem)
                 else:
                     counter = 0

dpnp/dpnp_iface.py

@@ -46,6 +46,8 @@
 import dpnp.config as config
 import collections
 
+import dpctl
+
 from dpnp.dpnp_algo import *
 from dpnp.dpnp_utils import *
 from dpnp.fft import *
@@ -134,8 +136,7 @@ def asnumpy(input, order='C'):
 
     """
     if config.__DPNP_OUTPUT_DPCTL__ and hasattr(input, "__sycl_usm_array_interface__"):
-        strides = (x * input.itemsize for x in input.strides)
-        return np_st.as_strided(input.usm_data.copy_to_host().view(input.dtype), shape=input.shape, strides=strides)
+        return dpctl.tensor.to_numpy(input)
 
     return numpy.asarray(input, order=order)