code: def test_warn_if_session_already_exists(self, mock_warn):
      # Set this to False, so Sessions created in previous tests do not trigger
      # the warning.
      mixed_precision_global_state.set_non_mixed_precision_session_created(
          False
      )
  
      with session.Session():
          mixed_precision.enable_mixed_precision_graph_rewrite_v1(
              gradient_descent_v1.GradientDescentOptimizer(1.0)
          )
          mock_warn.assert_any_call(
              "You already have existing Sessions that do not use mixed precision. "
              "enable_mixed_precision_graph_rewrite() will not affect these "
              "Sessions."
          )
  
imports: import os
  from absl.testing import parameterized
  from tensorflow.core.protobuf import config_pb2
  from tensorflow.python.client import session
  from tensorflow.python.eager import context
  from tensorflow.python.eager import def_function
  from tensorflow.python.framework import config
  from tensorflow.python.framework import test_util
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import math_ops
  from tensorflow.python.ops import variables
  from tensorflow.python.platform import test
  from tensorflow.python.platform import tf_logging
  from tensorflow.python.training import gradient_descent
  from tensorflow.python.training.experimental import loss_scale_optimizer
  from tensorflow.python.training.experimental import mixed_precision
  from tensorflow.python.training.experimental import mixed_precision_global_state
label_desc: Use image node_id 8 for calling the MixedPrecisionTest obj's underlying member method code with example usage: obj.test_warn_if_session_already_exists(mock_warn) without return types