docstr: Test setting the policy to None.
docstr_tok: ["Test","setting","the","policy","to","None","."]
code: def test_set_policy_none(self):
      """Test setting the policy to None."""
      with self.assertRaisesRegex(
          ValueError, "Invalid `policy` argument"
      ):
          set_dtype_policy(None)
  
imports: from keras.mixed_precision import DTypePolicy
  from keras.mixed_precision import dtype_policy
  from keras.mixed_precision import set_dtype_policy
  from keras.testing import test_case
label_desc: Use image node_id 3 for calling the DTypePolicyGlobalFunctionsEdgeCasesTest obj's underlying member method code with example usage: obj.test_set_policy_none() without return types