`FlaxBartForConditionalGeneration` should not require `input_ids` when `encoder_output` is provided
## Environment info

- `transformers` version: 4.18.0
- Platform: Linux-5.11.0-1018-gcp-x86_64-with-glibc2.31
- Python version: 3.10.4
- Huggingface_hub version: 0.5.1
- PyTorch version (GPU?): 1.11.0+cu102 (False)
- Tensorflow version (GPU?): 2.8.0 (False)
- Flax version (CPU?/GPU?/TPU?): 0.4.1 (cpu)
- Jax version: 0.3.5
- JaxLib version: 0.3.5
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help

@patil-suraj

## Information

Model I am using: BART

## To reproduce

```python
from transformers import BartTokenizer, BartForConditionalGeneration, FlaxBartForConditionalGeneration

tokenizer = BartTokenizer.from_pretrained('facebook/bart-base')
model_flax = FlaxBartForConditionalGeneration.from_pretrained('facebook/bart-base')
inputs = tokenizer('Travelers wait about an hour and a half to cross the Tower.', return_tensors='jax')
outputs_flax = model_flax.encode(**inputs)
generate_ids_flax = model_flax.generate(attention_mask=inputs.attention_mask, encoder_output=outputs_flax)  # TypeError: FlaxGenerationMixin.generate() missing 1 required positional argument: 'input_ids'

import numpy as onp
import torch
from transformers.modeling_outputs import BaseModelOutput

def jax2pt(a):
    return torch.from_numpy(onp.asarray(a))

model_pt = BartForConditionalGeneration.from_pretrained('facebook/bart-base')
outputs_pt = BaseModelOutput(last_hidden_state=jax2pt(outputs_flax.last_hidden_state))
generate_ids_pt = model_pt.generate(attention_mask=jax2pt(inputs.attention_mask), encoder_outputs=outputs_pt)
print(generate_ids_pt)  # OK
```

## Expected behavior

The Flax model should work as the PyTorch model.

## Actual behavior

`FlaxBartForConditionalGeneration` requires `input_ids`, even if `encoder_output` is provided.

ping @patil-suraj<|||||>Hey @ayaka14732 ! The flax generate method does not support passing in `encoder_outputs`, so `input_ids` is a required input.<|||||>Thanks! Why does the flax generate method not support passing in `encoder_outputs`? Is that a bug or a feature?

I am trying to generate with `encoder_outputs`, but I don't have the `input_ids`. That's because the `encoder_outputs` are produced from a customized encoder rather than the original one. What approach would you suggest to achieve this?<|||||>We can support passing `encoder_outputs` in flax generate. Would you like to open a PR for this ? Happy to help with it.

We'll need to modify this method to skip calling `.encode` if the `encoder_outputs` are passed as a kwarg.
https://github.com/huggingface/transformers/blob/bae9b6458cb4aebaf3a2eea1ab5d47904062f30f/src/transformers/generation_flax_utils.py#L142-L149<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>I am going to make a PR<|||||>@patil-suraj

Although the modification seems easy, I didn't find similar code in `generation_utils.py` to be used as a reference. In other words, the PyTorch version does not check `encoder_outputs` in `_prepare_encoder_decoder_kwargs_for_generation`:

https://github.com/huggingface/transformers/blob/b9bb417324c0d9013c505dc39c016ab9ca0e23c8/src/transformers/generation_utils.py#L507-L527

Why is that?<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>I believe this is not completed