Handle image_embeds in ViltModel
# What does this PR do?

Handle `image_embeds` in `ViltModel` / `ViltForImagesAndTextClassification`.

(looks like `Vilt` is the first model introducing `image_embeds` argument.)

## More Info

The `image_embeds` in `ViltForImagesAndTextClassification` should have `num_images` dimension as `pixel_values` has, as far as I understand.
_The documentation is not available anymore as the PR was closed or merged._<|||||>> Thanks for improving this!
> 
> Out of interest: were you experimenting with ViLT?

Not for this PR. I tried to fix a CI (vit-mae), which was about `test_torchscript`.
It turns out to be related to model main input -> I worked/improved on it -> more models involved including ViLT -> I just took this chance to work on this PR (otherwise I would forget it very quickly)