community[minor]: Implement Doctran async execution (#22372)

**Description**

The DoctranTextTranslator has an async transform function that was not
implemented because [the doctran
library](https://github.com/psychic-api/doctran) uses a sync version of
the `execute` method.

- I implemented the `DoctranTextTranslator.atransform_documents()`
method using `asyncio.to_thread` to run the function in a separate
thread.
- I updated the example in the Notebook with the new async version.
- The performance improvements can be appreciated when a big document is
divided into multiple chunks.

Relates to:
- Issue #14645: https://github.com/langchain-ai/langchain/issues/14645
- Issue #14437: https://github.com/langchain-ai/langchain/issues/14437
- https://github.com/langchain-ai/langchain/pull/15264

---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/community/langchain_community/document_transformers/doctran_text_translate.py

@@ -1,6 +1,10 @@
+from __future__ import annotations
+
+import asyncio
 from typing import Any, Optional, Sequence
 
 from langchain_core.documents import BaseDocumentTransformer, Document
+from langchain_core.runnables.config import run_in_executor
 from langchain_core.utils import get_from_env
 
 
@@ -36,10 +40,58 @@ class DoctranTextTranslator(BaseDocumentTransformer):
         )
         self.language = language
 
+    async def _aparse_document(
+        self, doctran: Any, index: int, doc: Document
+    ) -> tuple[int, Any]:
+        parsed_doc = await run_in_executor(
+            None, doctran.parse, content=doc.page_content, metadata=doc.metadata
+        )
+        return index, parsed_doc
+
+    async def _atranslate_document(
+        self, index: int, doc: Any, language: str
+    ) -> tuple[int, Any]:
+        translated_doc = await run_in_executor(
+            None, lambda: doc.translate(language=language).execute()
+        )
+        return index, translated_doc
+
     async def atransform_documents(
         self, documents: Sequence[Document], **kwargs: Any
     ) -> Sequence[Document]:
-        raise NotImplementedError
+        """Translates text documents using doctran."""
+        try:
+            from doctran import Doctran
+
+            doctran = Doctran(
+                openai_api_key=self.openai_api_key, openai_model=self.openai_api_model
+            )
+        except ImportError:
+            raise ImportError(
+                "Install doctran to use this parser. (pip install doctran)"
+            )
+
+        parse_tasks = [
+            self._aparse_document(doctran, i, doc) for i, doc in enumerate(documents)
+        ]
+        parsed_results = await asyncio.gather(*parse_tasks)
+
+        parsed_results.sort(key=lambda x: x[0])
+        doctran_docs = [doc for _, doc in parsed_results]
+
+        translate_tasks = [
+            self._atranslate_document(i, doc, self.language)
+            for i, doc in enumerate(doctran_docs)
+        ]
+        translated_results = await asyncio.gather(*translate_tasks)
+
+        translated_results.sort(key=lambda x: x[0])
+        translated_docs = [doc for _, doc in translated_results]
+
+        return [
+            Document(page_content=doc.transformed_content, metadata=doc.metadata)
+            for doc in translated_docs
+        ]
 
     def transform_documents(
         self, documents: Sequence[Document], **kwargs: Any