class_docstr: encode the bounding boxes
class_docstr_tok: ["encode","the","bounding","boxes"]
code: def bbox_delta(self, bbox_a, bbox_b):
      n = bbox_a.shape[0]
      result = nd.zeros((n, 4), ctx=bbox_a.context)
      result[:, 0] = bbox_a[:, 0] - bbox_b[:, 0]
      result[:, 1] = bbox_a[:, 1] - bbox_b[:, 1]
      result[:, 2] = nd.log(
          (bbox_a[:, 2] - bbox_a[:, 0] + 1e-8)
          / (bbox_b[:, 2] - bbox_b[:, 0] + 1e-8)
      )
      result[:, 3] = nd.log(
          (bbox_a[:, 3] - bbox_a[:, 1] + 1e-8)
          / (bbox_b[:, 3] - bbox_b[:, 1] + 1e-8)
      )
      return result
  
imports: import pickle
  import dgl
  import gluoncv
  import mxnet
  import numpy
  from dgl.nn.mxnet import GraphConv
  from dgl.utils import toindex
  from mxnet import nd
  from mxnet.gluon import nn
label_desc: Use image node_id 2 for calling the EdgeBBoxExtend obj's underlying member method code with example usage: obj.bbox_delta(bbox_a, bbox_b) and returns: result