// the tape, so no gradient needs to be computed from it unless this
        // target is also a source.
        auto source_tensor = sources_that_are_targets.find(id);
        if (source_tensor != sources_that_are_targets.end()) {
          Gradient* ones_like = nullptr;
          TF_RETURN_IF_ERROR(
              vspace.BuildOnesLike(source_tensor->second, &ones_like));
          (*result)[id].push_back(ones_like);
        }
      }

// and weren't used as input to any node in the imported graph def. The number
// of fetched mappings is returned in `num_missing_unused_input_mappings`. The
// array of each mapping's source node name is returned in `src_names`, and the
// array of each mapping's source index is returned in `src_indexes`.
//
// `*src_names`, `*src_indexes`, and the memory backing each string in
// `src_names` are owned by and have the lifetime of `results`.

  virtual void ClearCachesAndThreadExecutors() = 0;

  // Initialize the step resource container for a training step. This is used
  // in current TF runtime. For tfrt, it is used by fallback op handler.
  virtual void StartStep() = 0;
  // Destroy the step resource container for a training step.
  virtual void EndStep() = 0;