// Attempts to evaluate `output`. This will only be possible if `output` doesn't
// depend on any graph inputs (this function is safe to call if this isn't the
// case though).
//
// If the evaluation is successful, this function returns true and `output`s
// value is returned in `result`. Otherwise returns false. An error status is
// returned if something is wrong with the graph or input. Note that this may

*   Added experimental support for an exactly-once visitation guarantee for evaluating Keras models trained with `tf.distribute ParameterServerStrategy`, via the `exact_evaluation_shards` argument in `Model.fit` and `Model.evaluate`.