[Bug]: RankLLMRerank cannot be imported without vllm installed, regardless of reranking backend

Finding: In an external report, importing RankLLMRerank from llama-index-postprocessor-rankllm-rerank (current main) with rank-llm 0.25.7 fails with ModuleNotFoundError: No module named 'vllm' when vllm is not installed, even if the chosen reranking backend has nothing to do with vLLM. It was reported on macOS (Apple M4) with Python 3.12.5 (https://github.com/run-llama/llama_index/issues/23339). Reported mechanism (reporter analysis, not maintainer-confirmed): base.py imports PromptMode and Reranker from rank_llm.rerank at module level. In rank-llm 0.25.7 this reportedly executes rank_llm/rerank/__init__.py, which imports reranker.py, then rank_llm.rerank.listwise, then RankListwiseOSLLM, which does a bare import vllm. The reporter also says vllm's CUDA-only dependencies have no macOS wheels, so on Apple Silicon the package cannot be installed at all; the quoted install error mentions nvidia-cudnn-frontend lacking a distribution for the platform (same issue). The reporter notes that an earlier fix (#21789) changed the PromptMode import path and is not the cause, and that the regression test from that change mocks the rank_llm module tree, so the test suite avoids this production import path (same issue). Maintainer status: a collaborator commented that this needs an upstream fix (https://github.com/run-llama/llama_index/issues/23339#issuecomment-5954686479). The reporter replied that they would raise it with castorini/rank_llm, whose unreleased main branch reportedly wraps the Reranker import in try/except ImportError, and that a release with that guard would resolve the problem on the llama_index side without changes here (https://github.com/run-llama/llama_index/issues/23339#issuecomment-5954769792). The issue remained open at last update. Fix status: no merged fix or released version is identified in the supplied sources. The latest PyPI release is reportedly still 0.25.7. The reporter's idea of deferring rank_llm.rerank imports into _postprocess_nodes with a fallback is a proposal only and was not endorsed. Limitations: this is a single external report. It has not been reproduced by KFA. Reproduction is reported only on macOS/Python 3.12.5, and the upstream main-branch claim has not been independently verified.