patch[experimental] Fix start_index in SemanticChunker (#24761)

- Cause chunks are joined by space, so they can't be found in text, and
the final `start_index` is very possibility to be -1.
- The simplest way is to use the natural index of the chunk as
`start_index`.

libs/experimental/langchain_experimental/text_splitter.py

@@ -262,14 +262,14 @@ class SemanticChunker(BaseDocumentTransformer):
         _metadatas = metadatas or [{}] * len(texts)
         documents = []
         for i, text in enumerate(texts):
-            index = -1
+            start_index = 0
             for chunk in self.split_text(text):
                 metadata = copy.deepcopy(_metadatas[i])
                 if self._add_start_index:
-                    index = text.find(chunk, index + 1)
-                    metadata["start_index"] = index
+                    metadata["start_index"] = start_index
                 new_doc = Document(page_content=chunk, metadata=metadata)
                 documents.append(new_doc)
+                start_index += len(chunk)
         return documents
 
     def split_documents(self, documents: Iterable[Document]) -> List[Document]: