fix bug for trainer_seq2seq, generation_inputs should be a dict befor…
…e sending to model.generate (Arround Line 165 - Line 185)

# Fixing Bug

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Fixes # (issue)

In `trainer_seq2seq.py / Seq2SeqTrainer / prediction_step`, Line 174 - Line 177 read:

```python
generated_tokens = self.model.generate(
    **generation_inputs,
    **gen_kwargs,
)
```

which require the generated_tokens to be a `dict`. However, in the `else` branch in Line 171, the `generation_inputs` is created as a `Tensor` object, which will cause a problem.

Fix this by creating `generation_inputs` as a dict, and add a key called `input_ids`.