/** Singleton placeholder that indicates a value is being loaded. */
  @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
  static <K, V> ValueReference<K, V> unset() {
    return (ValueReference<K, V>) UNSET;
  }

  private enum NullEntry implements ReferenceEntry<Object, Object> {
    INSTANCE;

    @Override
    public @Nullable ValueReference<Object, Object> getValueReference() {

  /** Singleton placeholder that indicates a value is being loaded. */
  @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
  static <K, V> ValueReference<K, V> unset() {
    return (ValueReference<K, V>) UNSET;
  }

  private enum NullEntry implements ReferenceEntry<Object, Object> {
    INSTANCE;

    @Override
    public @Nullable ValueReference<Object, Object> getValueReference() {

    }

    @Override
    public Object getValue() {
      throw new AssertionError();
    }
  }

  /**
   * A singleton {@link WeakValueReference} used to denote an unset value in an entry with weak
   * values.
   */
  static final WeakValueReference<Object, Object, DummyInternalEntry> UNSET_WEAK_VALUE_REFERENCE =
      new WeakValueReference<Object, Object, DummyInternalEntry>() {

    }

    @Override
    public Object getValue() {
      throw new AssertionError();
    }
  }

  /**
   * A singleton {@link WeakValueReference} used to denote an unset value in an entry with weak
   * values.
   */
  static final WeakValueReference<Object, Object, DummyInternalEntry> UNSET_WEAK_VALUE_REFERENCE =
      new WeakValueReference<Object, Object, DummyInternalEntry>() {

    *   To explicitly enable or disable oneDNN optimizations, set the environment variable `TF_ENABLE_ONEDNN_OPTS` to `1` (enable) or `0` (disable) before running TensorFlow. To fall back to default settings, unset the environment variable.
    *   oneDNN optimizations can yield slightly different numerical results compared to when oneDNN optimizations are disabled due to floating-point round-off errors from