code: def expand_dims(self, axis):
          
          if axis == -1:
              axis = self.ndim
          if axis <= self._distaxis:
              subaxis = axis
              new_distaxis = self._distaxis + 1
          else:
              subaxis = axis - 1
              new_distaxis = self._distaxis
          new_subarrays = [expand_dims(ra, subaxis) for ra in self._subarrays]
          return DistArray(new_subarrays, new_distaxis)
docs: Insert a new axis, at a given position in the array shape
          Args:
            axis (int): Position (amongst axes) where new axis is to be inserted.