[LayoutLMv2] Update requires_backends of feature extractor
# What does this PR do?

This PR moves the `requires_backends` to the call method instead of the init of `LayoutLMv2FeatureExtractor`.

Fixes #15269
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15941). All of your documentation changes will be reflected on that endpoint.