[s2s] warn if --fp16 for torch 1.6 (#6977)

Author: sshleifer

Diff for: examples/seq2seq/finetune.py

@@ -3,13 +3,15 @@
 import logging
 import os
 import time
+import warnings
 from collections import defaultdict
 from pathlib import Path
 from typing import Dict, List, Tuple
 
 import numpy as np
 import pytorch_lightning as pl
 import torch
+from packaging import version
 from torch.utils.data import DataLoader
 
 from lightning_base import BaseTransformer, add_generic_args, generic_train
@@ -354,7 +356,8 @@ def main(args, model=None) -> SummarizationModule:
             model: SummarizationModule = SummarizationModule(args)
         else:
             model: SummarizationModule = TranslationModule(args)
-
+    if version.parse(torch.__version__) == version.parse("1.6") and args.fp16:
+        warnings.warn("FP16 only seems to work with torch 1.5+apex")
     dataset = Path(args.data_dir).name
     if (
         args.logger_name == "default"