Add multithread dataloder support.

wit/meaning_dataset.py

@@ -447,6 +447,14 @@ class BatchGroupMeaningDataloader(Dataset):
             s += "--------------------------------------------------------\n"
         return s
 
+    def detection_collate(batch):
+        return batch[0]
+
+    def dataloader(self, num_workers=1):
+        return DataLoader(
+            self, batch_size=1, num_workers=num_workers, collate_fn=BatchGroupMeaningDataloader.detection_collate
+        )
+
 
 if __name__ == "__main__":