message: Fix issue with tf.nn.relu6 grad in eager
  This fix tries to address the issue raised in 21380
  where the gradient of tf.nn.relu6 throws out an error.
  The issue seems to be that relu6 was not placed
  in OpDoesntRequireInput like tf.nn.relu.
  This fix fixes 21380.
diff: @@ -1726,7 +1726,6 @@ bool OpDoesntRequireOutput(const string& op_name) {
  "BiasAdd",
  "BiasAddV1",
  "BiasAddGrad",
  - "Relu6",
  "Softplus",
  "SoftplusGrad",
  "Softsign",
  @@ -1799,6 +1798,7 @@ bool OpDoesntRequireInput(const string& op_name) {
  "LogSoftmax",
  "BiasAdd",
  "Relu",
  + "Relu6",
  "Elu",
  "Selu",
  "SparseSoftmaxCrossEntropyWithLogits",
  