[chat] add zero2 cpu strategy for sft training (#3520)

applications/Chat/examples/train_sft.py

@@ -35,6 +35,8 @@ def train(args):
         strategy = ColossalAIStrategy(stage=3, placement_policy='cuda')
     elif args.strategy == 'colossalai_zero2':
         strategy = ColossalAIStrategy(stage=2, placement_policy='cuda')
+    elif args.strategy == 'colossalai_zero2_cpu':
+        strategy = ColossalAIStrategy(stage=2, placement_policy='cpu')
     else:
         raise ValueError(f'Unsupported strategy "{args.strategy}"')
 
@@ -168,7 +170,7 @@ def train(args):
 if __name__ == '__main__':
     parser = argparse.ArgumentParser()
     parser.add_argument('--strategy',
-                        choices=['naive', 'ddp', 'colossalai_gemini', 'colossalai_zero2'],
+                        choices=['naive', 'ddp', 'colossalai_gemini', 'colossalai_zero2', 'colossalai_zero2_cpu'],
                         default='naive')
     parser.add_argument('--model', choices=['gpt2', 'bloom', 'opt', 'llama'], default='bloom')
     parser.add_argument('--pretrain', type=str, default=None)