docstr: Test set_dtype_policy with a valid string.
docstr_tok: ["Test","set_dtype_policy","with","a","valid","string","."]
code: def test_set_dtype_policy_valid_string(self):
      """Test set_dtype_policy with a valid string."""
      set_dtype_policy("mixed_float16")
      policy = dtype_policy()
      self.assertEqual(policy.name, "mixed_float16")
  
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 2 for calling the DTypePolicyGlobalFunctionsTest obj's underlying member method code with example usage: obj.test_set_dtype_policy_valid_string() without return types