online DPO evaluation

### System Info

trl=0.11.2

### Information

- [ ] The official example scripts
- [ ] My own modified scripts

### Tasks

- [x] An officially supported task in the `examples` folder
- [ ] My own task or dataset (give details below)

### Reproduction

for [online dpo code](https://github.com/huggingface/trl/blob/47d08a962679ba0f6ca1b94c0902479525235512/examples/scripts/dpo_online.py#L16-L15)

If I add `--eval_steps=5` and `--eval_strategy=steps`, it shows error:
```
File "/mnt/task_runtime/trl/examples/scripts/dpo_online.py", line 120, in
trainer.train()
File "/opt/miniconda/lib/python3.9/site-packages/transformers/trainer.py", line 1938, in train
return inner_training_loop(
File "/opt/miniconda/lib/python3.9/site-packages/transformers/trainer.py", line 2356, in _inner_training_loop
self._maybe_log_save_evaluate(tr_loss, grad_norm, model, trial, epoch, ignore_keys_for_eval)
File "/mnt/task_runtime/trl/trl/trainer/online_dpo_trainer.py", line 555, in _maybe_log_save_evaluate
metrics = self._evaluate(trial, ignore_keys_for_eval)
File "/opt/miniconda/lib/python3.9/site-packages/transformers/trainer.py", line 2761, in _evaluate
metrics = self.evaluate(ignore_keys=ignore_keys_for_eval)
File "/opt/miniconda/lib/python3.9/site-packages/transformers/trainer.py", line 3666, in evaluate
output = eval_loop(
File "/opt/miniconda/lib/python3.9/site-packages/transformers/trainer.py", line 3857, in evaluation_loop
losses, logits, labels = self.prediction_step(model, inputs, prediction_loss_only, ignore_keys=ignore_keys)
File "/opt/miniconda/lib/python3.9/site-packages/transformers/trainer.py", line 4085, in prediction_step
outputs = model(**inputs)
File "/opt/miniconda/lib/python3.9/site-packages/torch/nn/modules/module.py", line 1553, in _wrapped_call_impl
return self._call_impl(*args, **kwargs)
File "/opt/miniconda/lib/python3.9/site-packages/torch/nn/modules/module.py", line 1562, in _call_impl
return forward_call(*args, **kwargs)
File "/opt/miniconda/lib/python3.9/site-packages/torch/nn/parallel/data_parallel.py", line 186, in forward
outputs = self.parallel_apply(replicas, inputs, module_kwargs)
File "/opt/miniconda/lib/python3.9/site-packages/torch/nn/parallel/data_parallel.py", line 201, in parallel_apply
return parallel_apply(replicas, inputs, kwargs, self.device_ids[:len(replicas)])
File "/opt/miniconda/lib/python3.9/site-packages/torch/nn/parallel/parallel_apply.py", line 109, in parallel_apply
output.reraise()
File "/opt/miniconda/lib/python3.9/site-packages/torch/_utils.py", line 706, in reraise
raise exception
TypeError: Caught TypeError in replica 0 on device 0.
Original Traceback (most recent call last):
File "/opt/miniconda/lib/python3.9/site-packages/torch/nn/parallel/parallel_apply.py", line 84, in _worker
output = module(*input, **kwargs)
File "/opt/miniconda/lib/python3.9/site-packages/torch/nn/modules/module.py", line 1553, in _wrapped_call_impl
return self._call_impl(*args, **kwargs)
File "/opt/miniconda/lib/python3.9/site-packages/torch/nn/modules/module.py", line 1562, in _call_impl
return forward_call(*args, **kwargs)
TypeError: forward() got an unexpected keyword argument 'prompt'
```

### Expected behavior

The eval is not using the correct input dataset.