class_docstr: Assign a corresponding gt bbox or background to each bbox.
  
  Each proposals will be assigned with `0` or a positive integer
  indicating the ground truth index.
  
  - 0: negative sample, no assigned gt
  - positive integer: positive sample, index (1-based) of assigned gt
  
  Args:
      topk (float): number of bbox selected in each level
code: def __init__(self, topk=9):
      self.topk = topk
  
file_path: PaddleDetection/ppdet/data/transform/atss_assigner.py
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import numpy
  from ppdet.utils.logger import setup_logger
label_desc: Use image node_id 1 to create a new ATSSAssigner object from inherited base classes: object with example: obj = ATSSAssigner(topk)