code: def test_warnings(self):
      with self.assertWarns(Warning):
          chn_input = torch.ones((1, 1, 3))
          chn_target = torch.ones((1, 1, 3))
          loss = FocalLoss(to_onehot_y=True)
          loss(chn_input, chn_target)
      with self.assertWarns(Warning):
          chn_input = torch.ones((1, 1, 3))
          chn_target = torch.ones((1, 1, 3))
          loss = FocalLoss(include_background=False)
          loss(chn_input, chn_target)
      with self.assertWarns(Warning):
          chn_input = torch.ones((1, 3, 3))
          chn_target = torch.ones((1, 3, 3))
          loss = FocalLoss(
              include_background=False, use_softmax=True, alpha=0.5
          )
          loss(chn_input, chn_target)
  
imports: from __future__ import annotations
  import unittest
  import numpy
  import torch
  import torch.nn
  import torch.nn.functional
  from parameterized import parameterized
  from monai.losses import FocalLoss
  from monai.networks import one_hot
  from tests.utils import test_script_save
label_desc: Use image node_id 15 for calling the TestFocalLoss obj's underlying member method code with example usage: obj.test_warnings() without return types