Return (sampler, n_samplers) tuples

def _sampler_n_samples(self, n_samples):
        """ Return (sampler, n_samplers) tuples """
        sampler_indices = self.rng_.choice(range(len(self.samplers)),
                                           size=n_samples,
                                           replace=True,
                                           p=self.weights)
        return [
            (self.samplers[idx], freq)
            for idx, freq in itemfreq(sampler_indices)
        ]