Add operation __index__

Author: antonwolfy

dpnp/dpnp_array.py

@@ -187,7 +187,10 @@ def __gt__(self, other):
  # '__imatmul__',
  # '__imod__',
  # '__imul__',
- # '__index__',
+ 
+     def __index__(self):
+        return self._array_obj.__index__()
+
  # '__init__',
  # '__init_subclass__',