code: def test_mpce_loss(self):
      costs = torch.tensor([[1, 0], [0, 1]])
      labels = torch.tensor([[1, 1], [0, 0]])
  
      loss = MultiPosCrossEntropyLoss(reduction="mean", loss_weight=1.0)
      assert torch.allclose(loss(costs, labels), torch.tensor(0.0))
  
      labels = torch.Tensor([[1, 0], [0, 1]])
      loss(costs, labels)
      assert torch.allclose(loss(costs, labels), torch.tensor(0.31326))
  
imports: from unittest import TestCase
  import torch
  from mmdet.models.losses import MultiPosCrossEntropyLoss
label_desc: Use image node_id 1 for calling the TestMultiPosCrossEntropyLoss obj's underlying member method code with example usage: obj.test_mpce_loss() without return types