add comment for pk sampler

ppcls/data/dataloader/pk_sampler.py

@@ -81,6 +81,7 @@ class PKSampler(DistributedBatchSampler):
     def __iter__(self):
         label_per_batch = self.batch_size // self.sample_per_label
         if self.shuffle:
+            # It's not accurate literally, but it helps in some dataset.
             np.random.RandomState(self.epoch).shuffle(self.label_list)
         for i in range(len(self)):
             batch_index = []