Add ONNX export w/ `optimum-onnx`

# Description

This PR adds the ONNX weights exported with [`optimum-onnx`](https://github.com/huggingface/optimum-onnx) as:

```python
from optimum.onnxruntime import ORTModelForCausalLM

model = ORTModelForCausalLM.from_pretrained("Roblox/roblox-pii-classifier", export=True)
model.save_pretrained("onnx/")
```

To be later on used via Python as follows (amongst many other options leveraging ONNX):

```python
from transformers import AutoTokenizer, pipeline
from optimum.onnxruntime import ORTModelForCausalLM

model = ORTModelForCausalLM.from_pretrained("Roblox/roblox-pii-classifier", revision="refs/pr/", subfolder="onnx/")
tokenizer = AutoTokenizer.from_pretrained("Roblox/roblox-pii-classifier", revision="refs/pr/")

pipe = pipeline("text-generation", model=model, tokenizer=tokenizer)
pipe("He never went out without a book under his arm")
```

Thanks 🤗

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