Allow the regular openai class to be used for ChatGPT models (#1393)

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

tests/integration_tests/llms/test_openai.py

@@ -144,6 +144,13 @@ async def test_openai_async_streaming_callback() -> None:
     assert isinstance(result, LLMResult)
 
 
+def test_openai_chat_wrong_class() -> None:
+    """Test OpenAIChat with wrong class still works."""
+    llm = OpenAI(model_name="gpt-3.5-turbo")
+    output = llm("Say foo:")
+    assert isinstance(output, str)
+
+
 def test_openai_chat() -> None:
     """Test OpenAIChat."""
     llm = OpenAIChat(max_tokens=10)