[rag] get_langchain_retriever supports local path (#3140)

# Description

Please add an informative description that covers that changes made by
the pull request and link all relevant issues.

# All Promptflow Contribution checklist:
- [x] **The pull request does not introduce [breaking changes].**
- [x] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
- [x] **I have read the [contribution guidelines](../CONTRIBUTING.md).**
- [ ] **Create an issue and link to the pull request to get dedicated
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## General Guidelines and Best Practices
- [x] Title of the pull request is clear and informative.
- [ ] There are a small number of commits, each of which have an
informative message. This means that previously merged commits do not
appear in the history of the PR. For more information on cleaning up the
commits in your PR, [see this
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### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

src/promptflow-rag/promptflow/rag/_get_langchain_retriever.py

@@ -1,14 +1,19 @@
 # ---------------------------------------------------------
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # ---------------------------------------------------------
+from pathlib import Path
 from azureml.rag.mlindex import MLIndex
 from promptflow.rag.constants._common import STORAGE_URI_TO_MLINDEX_PATH_FORMAT
 import re
+import yaml
 
 
 def get_langchain_retriever_from_index(path: str):
-    if not re.match(STORAGE_URI_TO_MLINDEX_PATH_FORMAT, path):
-        raise ValueError(
-            "Path to MLIndex file doesn't have the correct format."
-        )
-    return MLIndex(path).as_langchain_retriever()
+    if re.match(STORAGE_URI_TO_MLINDEX_PATH_FORMAT, path):
+        return MLIndex(path).as_langchain_retriever()
+
+    # local path
+    mlindex_path = str(Path(path) / "MLIndex") if not path.endswith("MLIndex") else path
+    with open(mlindex_path, "r") as f:
+        config = yaml.safe_load(f)
+        return MLIndex(mlindex_config=config).as_langchain_retriever()

src/promptflow-rag/requirements.txt

@@ -1,2 +1,2 @@
-azureml-rag[cognitive_search,document_parsing,langchain]
+azureml-rag[azure,cognitive_search,document_parsing,langchain]
 openai