Fix tests without any real effect in EncoderDecoderMixin
# What does this PR do?

In `test_modeling_encoder_decoder.py`, there are 2 places like

```
enc_dec_model.save_pretrained(tmpdirname)
EncoderDecoderModel.from_pretrained(tmpdirname)

after_outputs = enc_dec_model(...)
```
Therefore `after_outputs` will be exactly the same `outputs`, since `enc_dec_model` doesn't get the new value.
For the testing purpose, we need to do
```
enc_dec_model = EncoderDecoderModel.from_pretrained(tmpdirname)
```
(Hope I get it right)

## Who can review?

@patrickvonplaten @sgugger 

Thanks a lot @ydshieh !<|||||>The model should have been cast to the `torch_device` as otherwise the inputs/model are on different devices :)

https://github.com/huggingface/transformers/runs/3524847938?check_suite_focus=true<|||||>I'll take care of it :-) <|||||>Hey, thank you guys! Sorry about the device issue, I am less familiar with PyTorch, and it seems the checks I had when I opened this PR don't have that test above (probably it only runs when we merge to master?)<|||||>Don't worry about it @ydshieh :-)