Harrison/add huggingface hub (#23)

Add support for huggingface hub

I could not find a good way to enforce stop tokens over the huggingface
hub api - that needs to hopefully be cleaned up in the future

tests/integration_tests/llms/test_huggingface_hub.py

@@ -0,0 +1,19 @@
+"""Test HuggingFace API wrapper."""
+
+import pytest
+
+from langchain.llms.huggingface_hub import HuggingFaceHub
+
+
+def test_huggingface_call() -> None:
+    """Test valid call to HuggingFace."""
+    llm = HuggingFaceHub(max_new_tokens=10)
+    output = llm("Say foo:")
+    assert isinstance(output, str)
+
+
+def test_huggingface_call_error() -> None:
+    """Test valid call to HuggingFace that errors."""
+    llm = HuggingFaceHub(max_new_tokens=-1)
+    with pytest.raises(ValueError):
+        llm("Say foo:")