message: Improve the latency of small `tf.concat()` operations.
  Avoid copying each input `Tensor` object from the `OpInputList`, and
  instead take a const reference. This avoids the need to run the
  `Tensor` destructor at the end of each iteration.
diff: @@ -113,7 +113,7 @@ class ConcatBaseOp : public OpKernel {
  int64 output_concat_dim = 0;
  const bool input_is_scalar = IsLegacyScalar(input_shape);
  for (int i = 0; i < N; ++i) {
  - const auto in = values[i];
  + const auto& in = values[i];
  const bool in_is_scalar = IsLegacyScalar(in.shape());
  OP_REQUIRES(
  c, in.dims() == input_dims || (input_is_scalar && in_is_scalar),
  