// If false, all activations are deleted in the first call to ComputeGradient.
  // Else, only when this is destructed.
  bool persistent_;
};

// Describes a callback for special-cased and more efficient jvp computation.
//
// Could just be a simple typedef in ForwardAccumulator, but MSVC chokes on
// that.
template <typename Gradient>
class ForwardFunction

//                           If set to 0, the function's name will be `fn_name`.
//  num_opers - `num_opers` contains the number of elements in the `opers` array
//              or a special value of -1 meaning that no array is given.
//              The distinction between an empty array of operations and no
//              array of operations is necessary to distinguish the case of