Parameters:
      input ("Tensor") -- the tensor that represents the values of
      the function
Keyword Arguments:
      * spacing ("scalar", "list of scalar", "list of Tensor",
        optional) -- "spacing" can be used to modify how the "input"
        tensor's indices relate to sample coordinates. If "spacing" is
        a scalar then the indices are multiplied by the scalar to
        produce the coordinates. For example, if "spacing=2" the
        indices (1, 2, 3) become coordinates (2, 4, 6). If "spacing"
        is a list of scalars then the corresponding indices are
        multiplied. For example, if "spacing=(2, -1, 3)" the indices
        (1, 2, 3) become coordinates (2, -2, 9). Finally, if "spacing"
        is a list of one-dimensional tensors then each tensor
        specifies the coordinates for the corresponding dimension. For
        example, if the indices are (1, 2, 3) and the tensors are (t0,
        t1, t2), then the coordinates are (t0[1], t1[2], t2[3])