class_docstr: Affine warp constraints class.
  
  `AffineWarpConstraints` allow for
  very succinct definitions of constraints on
  the values of entries in affine transform matrices.
docstr: Computes number of non constrained parameters.
docstr_tok: ["Computes","number","of","non","constrained","parameters","."]
code: def _calc_num_free_params(self):
      """Computes number of non constrained parameters."""
      return sum(row.count(None) for row in 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 2 for calling the AffineWarpConstraints obj's underlying member method code with example usage: obj._calc_num_free_params() and returns: sum