code: def concatenate(box_mask_lists, fields=None):
      """Concatenate list of box_mask_lists.
  
      This op concatenates a list of input box_mask_lists into a larger
      box_mask_list.  It also
      handles concatenation of box_mask_list fields as long as the field tensor
      shapes are equal except for the first dimension.
  
      Args:
        box_mask_lists: list of np_box_mask_list.BoxMaskList objects
        fields: optional list of fields to also concatenate.  By default, all
          fields from the first BoxMaskList in the list are included in the
          concatenation.
  
      Returns:
        a box_mask_list with number of boxes equal to
          sum([box_mask_list.num_boxes() for box_mask_list in box_mask_list])
      Raises:
        ValueError: if box_mask_lists is invalid (i.e., is not a list, is empty, or
          contains non box_mask_list objects), or if requested fields are not
          contained in all box_mask_lists
      """
      if fields is not None:
          if "masks" not in fields:
              fields.append("masks")
      return box_list_to_box_mask_list(
          np_box_list_ops.concatenate(
              boxlists=box_mask_lists, fields=fields
          )
      )
  
file_path: SlowFast/ava_evaluation/np_box_mask_list_ops.py
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import numpy
  from .None import np_box_list_ops, np_box_mask_list, np_mask_ops
label_desc: Use image node_id 11 for calling a global function with example usage: concatenate(box_mask_lists, fields) and returns: box_list_to_box_mask_list