'Returns subsample of entries in indicator.
Args:
indicator: boolean tensor of shape [N] whose True entries can be sampled.
batch_size: desired batch size.
**params: additional keyword arguments for specific implementations of
the MinibatchSampler.
Returns:
sample_indicator: boolean tensor of shape [N] whose True entries have been
sampled. If sum(indicator) >= batch_size, sum(is_sampled) = batch_size'

@abstractmethod
def subsample(self, indicator, batch_size, **params):

    pass
