Unpin detectron2
# What does this PR do?

Unpin `detectron2` that are introduced in #18701 and #18680, as the issue was fixed on its side

https://github.com/facebookresearch/detectron2/commit/3cfda4704cb3b23255f29d683cf05cbaf10daaa4

The test `run_tests_layoutlmv2_and_v3` is OK now:

https://app.circleci.com/pipelines/github/huggingface/transformers/46171/workflows/c82c09a9-f6f7-4e21-8732-6681083960ff/jobs/541901
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_18727). All of your documentation changes will be reflected on that endpoint.