// operations and may be fetched by calling FetchJVP. ForwardAccumulator
  // maintains a reference to these JVPs: if an `output_tensors` Tensor is
  // deleted, `DeleteGradient` should be called as soon as possible to free the
  // (now inaccessible) corresponding JVPs, but ForwardAccumulator's destructor
  // will release remaining references.
  //
  // This method is not thread-safe (and in general ForwardAccumulator is not