class_docstr: Assign object detection proposals to ground-truth targets. Produces proposal
  classification labels and bounding-box regression targets.
class_docstr_tok: ["Assign","object","detection","proposals","to","ground-truth","targets",".","Produces","proposal","classification","labels","and","bounding-box","regression","targets","."]
docstr: This layer does not propagate gradients.
docstr_tok: ["This","layer","does","not","propagate","gradients","."]
code: def backward(self, state, root_gradients, variables):
      """This layer does not propagate gradients."""
      pass
  
file_path: CNTK/Examples/Image/Detection/utils/rpn/proposal_target_layer.py
imports: from cntk import output_variable, FreeDimension
  from cntk.ops.functions import UserFunction
  import yaml
  import numpy
  import numpy.random
  from utils.rpn.bbox_transform import bbox_transform
  from utils.cython_modules.cython_bbox import bbox_overlaps
label_desc: Use image node_id 4 for calling the ProposalTargetLayer obj's underlying member method code with example usage: obj.backward(state, root_gradients, variables) without return types