Add ViT export support and image classification

[JingyaHuang]

What does this PR do?

Export

• CLI

optimum-cli export neuron --model facebook/deit-base-patch16-224 --batch_size 1 --task image-classification vit_neuron/

• API

import requests
from PIL import Image
from transformers import AutoImageProcessor
from optimum.neuron import NeuronModelForImageClassification

# Create the feature extractor and model
model_id = "facebook/deit-base-patch16-224"
feature_extractor = AutoImageProcessor.from_pretrained(model_id)
model = NeuronModelForImageClassification.from_pretrained(model_id, export=True, batch_size=1)
save_directory = "vit_neuron"
model.save_pretrained(save_directory)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[dacorvo]

LGTM, thanks four for that neat pull-request !