!7149 add exception for mixed precision when device is CPU

Merge pull request !7149 from wukesong/lenet-mix-fix

model_zoo/official/cv/lenet/train.py

@@ -60,6 +60,10 @@ if __name__ == "__main__":
     config_ck = CheckpointConfig(save_checkpoint_steps=cfg.save_checkpoint_steps,
                                  keep_checkpoint_max=cfg.keep_checkpoint_max)
     ckpoint_cb = ModelCheckpoint(prefix="checkpoint_lenet", directory=args.ckpt_path, config=config_ck)
+
+    if args.device_target == "CPU":
+        model = Model(network, net_loss, net_opt, metrics={"Accuracy": Accuracy()})
+    else:
         model = Model(network, net_loss, net_opt, metrics={"Accuracy": Accuracy()}, amp_level="O2")
 
     print("============== Starting Training ==============")