### What problem does this PR solve?
MinerU supports VLM-Transfomers backend.
Set `MINERU_BACKEND="pipeline"` to choose the backend. (Options:
pipeline | vlm-transformers, default is pipeline)
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
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Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>