Fix SegformerForImageClassification
# What does this PR do?

After #15889 and removing the deprecated `reshape_last_stage` attribute for 6 checkpoints on the hub, this PR makes sure `SegformerForImageClassification` properly uses the features from the backbone in order to compute the image class.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15895). All of your documentation changes will be reflected on that endpoint.