docstr: Test the length of detection instance results is 0.
code: def test_bbox_post_process(self):
      """Test the length of detection instance results is 0."""
      from mmengine.config import ConfigDict
  
      cfg = ConfigDict(
          nms_pre=1000,
          max_per_img=1000,
          nms=dict(type="nms", iou_threshold=0.7),
          min_bbox_size=0,
      )
  
      rpn_head = RPNHead(num_classes=1, in_channels=1)
      results = InstanceData(metainfo=dict())
      results.bboxes = torch.zeros((0, 4))
      results.scores = torch.zeros(0)
      results = rpn_head._bbox_post_process(
          results, cfg, img_meta=dict()
      )
      self.assertEqual(len(results), 0)
      self.assertEqual(results.bboxes.size(), (0, 4))
      self.assertEqual(results.scores.size(), (0,))
      self.assertEqual(results.labels.size(), (0,))
  
file_path: mmdetection/tests/test_models/test_dense_heads/test_rpn_head.py
imports: from unittest import TestCase
  import pytest
  import torch
  from mmengine import Config
  from mmengine.structures import InstanceData
  from mmdet import *
  from mmdet.models.dense_heads import RPNHead
label_desc: Use image node_id 3 for calling the TestRPNHead obj's underlying member method code with example usage: obj.test_bbox_post_process() without return types