RandomIdentitySampler: n_pids should be larger than n_pids_to_sample

torchreid/data/sampler.py

@@ -36,6 +36,7 @@ def __init__(self, data_source, batch_size, num_instances):
             pid = items[1]
             self.index_dic[pid].append(index)
         self.pids = list(self.index_dic.keys())
+        assert len(self.pids) >= self.num_pids_per_batch
 
         # estimate number of examples in an epoch
         # TODO: improve precision