[seq2seq] Don't copy self.source in sortishsampler (#5818)

examples/seq2seq/utils.py

@@ -144,16 +144,9 @@ class SummarizationDataset(Dataset):
         batch = {"input_ids": source_ids, "attention_mask": source_mask, "decoder_input_ids": y}
         return batch
 
-    @property
-    def src_lens(self):  # Can delete?
-        return lmap(len, self.source)
-
-    @property
-    def tgt_lens(self):
-        return lmap(len, self.target)
-
     def make_sortish_sampler(self, batch_size):
-        return SortishSampler(self.source, batch_size)
+        lens = [x["input_ids"].ne(self.pad_token_id).sum() for x in self.source]
+        return SortishSampler(lens, batch_size)
 
 
 class SortishSampler(Sampler):
@@ -163,7 +156,7 @@ class SortishSampler(Sampler):
         self.data, self.bs = data, batch_size
 
     def key(self, i):
-        return len(self.data[i])
+        return self.data[i]
 
     def __len__(self) -> int:
         return len(self.data)