feat: ctransformers support async chain (#6859)

- Description: Adding async method for CTransformers 
- Issue: I've found impossible without this code to run Websockets
inside a FastAPI micro service and a CTransformers model.
  - Tag maintainer: Not necessary yet, I don't like to mention directly 
  - Twitter handle: @_semoal

tests/integration_tests/llms/test_ctransformers.py

@@ -1,4 +1,5 @@
 """Test C Transformers wrapper."""
+import pytest
 
 from langchain.llms import CTransformers
 from tests.unit_tests.callbacks.fake_callback_handler import FakeCallbackHandler
@@ -19,3 +20,20 @@ def test_ctransformers_call() -> None:
     assert isinstance(output, str)
     assert len(output) > 1
     assert 0 < callback_handler.llm_streams <= config["max_new_tokens"]
+
+
+@pytest.mark.asyncio
+async def test_ctransformers_async_inference() -> None:
+    config = {"max_new_tokens": 5}
+    callback_handler = FakeCallbackHandler()
+
+    llm = CTransformers(
+        model="marella/gpt-2-ggml",
+        config=config,
+        callbacks=[callback_handler],
+    )
+
+    output = await llm._acall(prompt="Say foo:")
+    assert isinstance(output, str)
+    assert len(output) > 1
+    assert 0 < callback_handler.llm_streams <= config["max_new_tokens"]