add text2text generation (#93)

fixes issue #90

tests/integration_tests/llms/test_huggingface_hub.py

@@ -5,15 +5,22 @@ 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)
+def test_huggingface_text_generation() -> None:
+    """Test valid call to HuggingFace text generation model."""
+    llm = HuggingFaceHub(repo_id="gpt2", model_kwargs={"max_new_tokens": 10})
     output = llm("Say foo:")
     assert isinstance(output, str)
 
 
+def test_huggingface_text2text_generation() -> None:
+    """Test valid call to HuggingFace text2text model."""
+    llm = HuggingFaceHub(repo_id="google/flan-t5-xl")
+    output = llm("The capital of New York is")
+    assert output == "Albany"
+
+
 def test_huggingface_call_error() -> None:
     """Test valid call to HuggingFace that errors."""
-    llm = HuggingFaceHub(max_new_tokens=-1)
+    llm = HuggingFaceHub(model_kwargs={"max_new_tokens": -1})
     with pytest.raises(ValueError):
         llm("Say foo:")