feat: add loader for rich text files (#3227)

### Summary

Adds a loader for rich text files. Requires `unstructured>=0.5.12`.

### Testing

The following test uses the example RTF file from the [`unstructured`
repo](https://github.com/Unstructured-IO/unstructured/tree/main/example-docs).

```python
from langchain.document_loaders import UnstructuredRTFLoader

loader = UnstructuredRTFLoader("fake-doc.rtf", mode="elements")
docs = loader.load()
docs[0].page_content
```

langchain/document_loaders/__init__.py

@@ -57,6 +57,7 @@ from langchain.document_loaders.pdf import (
 from langchain.document_loaders.powerpoint import UnstructuredPowerPointLoader
 from langchain.document_loaders.readthedocs import ReadTheDocsLoader
 from langchain.document_loaders.roam import RoamLoader
+from langchain.document_loaders.rtf import UnstructuredRTFLoader
 from langchain.document_loaders.s3_directory import S3DirectoryLoader
 from langchain.document_loaders.s3_file import S3FileLoader
 from langchain.document_loaders.sitemap import SitemapLoader
@@ -106,6 +107,7 @@ __all__ = [
     "OutlookMessageLoader",
     "UnstructuredEPubLoader",
     "UnstructuredMarkdownLoader",
+    "UnstructuredRTFLoader",
     "RoamLoader",
     "YoutubeLoader",
     "S3FileLoader",

langchain/document_loaders/rtf.py

@@ -0,0 +1,28 @@
+"""Loader that loads rich text files."""
+from typing import Any, List
+
+from langchain.document_loaders.unstructured import (
+    UnstructuredFileLoader,
+    satisfies_min_unstructured_version,
+)
+
+
+class UnstructuredRTFLoader(UnstructuredFileLoader):
+    """Loader that uses unstructured to load rtf files."""
+
+    def __init__(
+        self, file_path: str, mode: str = "single", **unstructured_kwargs: Any
+    ):
+        min_unstructured_version = "0.5.12"
+        if not satisfies_min_unstructured_version(min_unstructured_version):
+            raise ValueError(
+                "Partitioning rtf files is only supported in "
+                f"unstructured>={min_unstructured_version}."
+            )
+
+        super().__init__(file_path=file_path, mode=mode, **unstructured_kwargs)
+
+    def _get_elements(self) -> List:
+        from unstructured.partition.rtf import partition_rtf
+
+        return partition_rtf(filename=self.file_path, **self.unstructured_kwargs)