class_docstr: Grid warper interface class.
  
  An object implementing the `GridWarper` interface
  generates a reference grid of feature points at construction time,
  and warps it via a parametric transformation model,
  specified at run time by an input parameter Tensor.
  Grid warpers must then implement a `create_features` function
  used to generate the reference grid to be warped
  in the forward pass (according to a determined warping model).
docstr: Returns a tuple containing the shape of the output grid.
docstr_tok: ["Returns","a","tuple","containing","the","shape","of","the","output","grid","."]
code: def output_shape(self):
      """Returns a tuple containing the shape of the output grid."""
      return self._output_shape
  
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 7 for calling the GridWarperLayer obj's underlying member method code with example usage: obj.output_shape() and returns: self