[Fix doc example] missing import (#15240)

* fix import

* fix style
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

src/transformers/models/encoder_decoder/modeling_tf_encoder_decoder.py

@@ -259,6 +259,8 @@ class TFEncoderDecoderModel(TFPreTrainedModel):
 
         ```python
         >>> # a workaround to load from pytorch checkpoint
+        >>> from transformers import EncoderDecoderModel, TFEncoderDecoderModel
+
         >>> _model = EncoderDecoderModel.from_pretrained("patrickvonplaten/bert2bert-cnn_dailymail-fp16")
         >>> _model.encoder.save_pretrained("./encoder")
         >>> _model.decoder.save_pretrained("./decoder")