message: Register a dummy estimation function for _Send ops. This ensures that we don't get negative cost estimates for _Send ops.
diff: @@ -35,6 +35,7 @@ constexpr char kRefIdentity[] = "RefIdentity";
  constexpr char kNoOp[] = "NoOp";
  constexpr char kReshape[] = "Reshape";
  constexpr char kRecv[] = "_Recv";
  +constexpr char kSend[] = "_Send";
  constexpr char kBatchMatMul[] = "BatchMatMul";
  constexpr char kVariable[] = "Variable";
  constexpr char kVariableV2[] = "VariableV2";
  @@ -165,6 +166,7 @@ OpLevelCostEstimator::OpLevelCostEstimator() {
  {kNoOp, wrap(&OpLevelCostEstimator::PredictNoOp)},
  {kReshape, wrap(&OpLevelCostEstimator::PredictNoOp)},
  {kRecv, wrap(&OpLevelCostEstimator::PredictNoOp)},
  + {kSend, wrap(&OpLevelCostEstimator::PredictNoOp)},
  {kVariable, wrap(&OpLevelCostEstimator::PredictNoOp)},
  {kVariableV2, wrap(&OpLevelCostEstimator::PredictNoOp)},
  {kBatchMatMul, wrap(&OpLevelCostEstimator::PredictBatchMatMul)},
  