'Evaluates and accumulates evaluation metric on outputs of the last forward
computation.
Parameters
eval_metric : EvalMetric
Evaluation metric to use.
labels : list of NDArray
Typically `data_batch.label`.
Examples
>>> # An example of updating evaluation metric.
>>> mod.forward(data_batch)
>>> mod.update_metric(metric, data_batch.label)'

def update_metric(self, eval_metric, labels):

    raise NotImplementedError()
