TensorFlow operations were converted into may have changed.
    *   Code that uses `tf.map_fn`/`tf.cond`/`tf.while_loop`/control flow as op
        layers and happens to work before TF 2.4. These will explicitly be
        unsupported now. Converting these ops to Functional API op layers was
        unreliable before TF 2.4, and prone to erroring incomprehensibly or

  if (input_tensors != nullptr) {
    // Note that we take the address of the elements in `all_input_tensors`
    // below. Allocate enough space so that no reallocation happens, which will
    // make the pointers invalid.
    all_input_tensors.reserve(num_inputs);
    for (int i = 0; i < num_inputs; ++i) {
      if (input_tensors[i] == nullptr) continue;
      all_input_tensors.emplace_back();

  // TODO(apassos) consider narrowing down this interface.
  BackwardFunction* backward_function;

  // Should be called before deleting the backward function. TODO(apassos) use
  // unique_ptrs to ensure this happens.
  std::function<void(BackwardFunction*)> backward_function_deleter;
};

// Map from tensor_id to internally-defined operation-id of the operation which