class_docstr: Affine Grid Warper class.
  
  The affine grid warper generates a reference grid of n-dimensional points
  and warps it via an affine transformation model determined by an input
  parameter Tensor. Some of the transformation parameters can be fixed at
  construction time via an `AffineWarpConstraints` object.
code: def constraints(self):
      return self._constraints
  
imports: from __future__ import absolute_import, division, print_function
  from itertools import chain
  import numpy
  import tensorflow
  from niftynet.layer.base_layer import Layer, LayerFromCallable, Invertible
label_desc: Use image node_id 3 for calling the AffineGridWarperLayer obj's underlying member method code with example usage: obj.constraints() and returns: self