fix TFMarianMTModel output
# What does this PR do?

One line change:

Current `TFMarianMTModel` returns `decoder`'s `last_hidden_state` as `encoder_last_hidden_state`. This PR fixes it.

```
encoder_last_hidden_state=outputs.last_hidden_state,  # index 0 of encoder outputs
```
_The documentation is not available anymore as the PR was closed or merged._