docstr: Test corner case where an image might have no true detections.
code: def test_atss_assigner_with_empty_gt(self):
      """Test corner case where an image might have no true detections."""
      atss_assigner = ATSSAssigner(topk=9)
      priors = torch.FloatTensor(
          [
              [0, 0, 10, 10],
              [10, 10, 20, 20],
              [5, 5, 15, 15],
              [32, 32, 38, 42],
          ]
      )
      gt_bboxes = torch.empty(0, 4)
      gt_labels = torch.empty(0)
  
      pred_instances = InstanceData(priors=priors)
      gt_instances = InstanceData(bboxes=gt_bboxes, labels=gt_labels)
      num_level_bboxes = [4]
      assign_result = atss_assigner.assign(
          pred_instances, num_level_bboxes, gt_instances
      )
  
      expected_gt_inds = torch.LongTensor([0, 0, 0, 0])
      self.assertTrue(
          torch.all(assign_result.gt_inds == expected_gt_inds)
      )
  
file_path: mmdetection/tests/test_models/test_task_modules/test_assigners/test_atss_assigner.py
imports: from unittest import TestCase
  import torch
  from mmengine.structures import InstanceData
  from mmdet.models.task_modules.assigners import ATSSAssigner
label_desc: Use image node_id 3 for calling the TestATSSAssigner obj's underlying member method code with example usage: obj.test_atss_assigner_with_empty_gt() without return types