langchain: add RankLLM Reranker (#21171)

Integrate RankLLM reranker (https://github.com/castorini/rank_llm) into
LangChain

An example notebook is given in
`docs/docs/integrations/retrievers/rankllm-reranker.ipynb`

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

libs/community/tests/integration_tests/document_compressors/__init__.py

@@ -0,0 +1 @@
+"""Test document compressor integrations."""

libs/community/tests/integration_tests/document_compressors/test_rankllm_rerank.py

@@ -0,0 +1,8 @@
+"""Test rankllm reranker."""
+
+from langchain_community.document_compressors.rankllm_rerank import RankLLMRerank
+
+
+def test_rankllm_reranker_init() -> None:
+    """Test the RankLLM reranker initializes correctly."""
+    RankLLMRerank()

libs/community/tests/unit_tests/document_compressors/test_imports.py

@@ -4,6 +4,7 @@ EXPECTED_ALL = [
     "LLMLinguaCompressor",
     "OpenVINOReranker",
     "JinaRerank",
+    "RankLLMRerank",
     "FlashrankRerank",
 ]