Add C Transformers for GGML Models (#5218)

# Add C Transformers for GGML Models
I created Python bindings for the GGML models:
https://github.com/marella/ctransformers

Currently it supports GPT-2, GPT-J, GPT-NeoX, LLaMA, MPT, etc. See
[Supported
Models](https://github.com/marella/ctransformers#supported-models).


It provides a unified interface for all models:

```python
from langchain.llms import CTransformers

llm = CTransformers(model='/path/to/ggml-gpt-2.bin', model_type='gpt2')

print(llm('AI is going to'))
```

It can be used with models hosted on the Hugging Face Hub:

```py
llm = CTransformers(model='marella/gpt-2-ggml')
```

It supports streaming:

```py
from langchain.callbacks.streaming_stdout import StreamingStdOutCallbackHandler

llm = CTransformers(model='marella/gpt-2-ggml', callbacks=[StreamingStdOutCallbackHandler()])
```

Please see [README](https://github.com/marella/ctransformers#readme) for
more details.
---------

Co-authored-by: Dev 2049 <dev.dev2049@gmail.com>

tests/integration_tests/llms/test_ctransformers.py

@@ -0,0 +1,21 @@
+"""Test C Transformers wrapper."""
+
+from langchain.llms import CTransformers
+from tests.unit_tests.callbacks.fake_callback_handler import FakeCallbackHandler
+
+
+def test_ctransformers_call() -> None:
+    """Test valid call to C Transformers."""
+    config = {"max_new_tokens": 5}
+    callback_handler = FakeCallbackHandler()
+
+    llm = CTransformers(
+        model="marella/gpt-2-ggml",
+        config=config,
+        callbacks=[callback_handler],
+    )
+
+    output = llm("Say foo:")
+    assert isinstance(output, str)
+    assert len(output) > 1
+    assert 0 < callback_handler.llm_streams <= config["max_new_tokens"]