code: def testIdentifyGradientRaisesLookupErrorForUnknownXTensor(self):
      grad_debugger_1 = debug_gradients.GradientsDebugger()
      grad_debugger_2 = debug_gradients.GradientsDebugger()
      id_grad_w = grad_debugger_1.identify_gradient(self.w)
      y = math_ops.add(id_grad_w, -1.0, name="y")
  
      # There are >1 gradient debuggers registered, and grad_debugger is not used
      # as a context manager here, so the gradient w.r.t. self.w will not be
      # registered.
      gradients_impl.gradients(y, [self.u, self.v])
  
      with self.assertRaisesRegex(
          LookupError,
          r"This GradientsDebugger has not received any gradient tensor for ",
      ):
          grad_debugger_1.gradient_tensor(self.w)
      with self.assertRaisesRegex(
          LookupError,
          r"This GradientsDebugger has not received any gradient tensor for ",
      ):
          grad_debugger_2.gradient_tensor(self.w)
  
imports: import tempfile
  from tensorflow.core.protobuf import config_pb2
  from tensorflow.core.protobuf import rewriter_config_pb2
  from tensorflow.python.client import session
  from tensorflow.python.debug.lib import debug_data
  from tensorflow.python.debug.lib import debug_gradients
  from tensorflow.python.debug.lib import debug_utils
  from tensorflow.python.framework import ops
  from tensorflow.python.framework import tensor
  from tensorflow.python.framework import test_util
  from tensorflow.python.lib.io import file_io
  from tensorflow.python.ops import gradients_impl
  from tensorflow.python.ops import math_ops
  from tensorflow.python.ops import variables
  from tensorflow.python.platform import googletest
  from tensorflow.python.training import gradient_descent
label_desc: Use image node_id 7 for calling the IdentifyGradientTest obj's underlying member method code with example usage: obj.testIdentifyGradientRaisesLookupErrorForUnknownXTensor() without return types