class_docstr: Box collection.
  
  BoxList represents a list of bounding boxes as numpy array, where each
  bounding box is represented as a row of 4 numbers,
  [y_min, x_min, y_max, x_max].  It is assumed that all bounding boxes within a
  given list correspond to a single image.
  
  Optionally, users can add additional related fields (such as
  objectness/classification scores).
docstr: Return number of boxes held in collections.
code: def num_boxes(self):
      """Return number of boxes held in collections."""
      return self.data["boxes"].shape[0]
  
file_path: SlowFast/ava_evaluation/np_box_list.py
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import numpy
label_desc: Use image node_id 2 for calling the BoxList obj's underlying member method code with example usage: obj.num_boxes() and returns: self