fix double wrapping + test (#10583)

src/transformers/trainer.py

@@ -738,6 +738,10 @@ class Trainer:
         if self.deepspeed:
             return self.deepspeed
 
+        # train/eval could be run multiple-times - if already wrapped, don't re-wrap it again
+        if unwrap_model(model) is not model:
+            return model
+
         # Mixed precision training with apex (torch < 1.6)
         if self.use_apex and training:
             model, self.optimizer = amp.initialize(model, self.optimizer, opt_level=self.args.fp16_opt_level)