BlockingQueue<E> q, Collection<? super E> buffer, int numElements, Duration timeout)
      throws InterruptedException {
    return drain(q, buffer, numElements, toNanosSaturated(timeout), NANOSECONDS);
  }

  /**
   * Drains the queue as {@link BlockingQueue#drainTo(Collection, int)}, but if the requested {@code
   * numElements} elements are not available, it will wait for them up to the specified timeout.
   *

 public:
  explicit TapeVSpace(AbstractContext* ctx) : ctx_(ctx) {}
  ~TapeVSpace() override {}

  // Returns the number of elements in the gradient tensor.
  int64_t NumElements(AbstractTensorHandle* tensor) const override;

  // Consumes references to the tensors in the gradient_tensors list and returns
  // a tensor with the result.
  AbstractTensorHandle* AggregateGradients(