Add validation for model_name parameter (#476)

This PR introduces a validation check for the `model_name` parameter
within the `openai` method of the `outlines.models` module. The aim is
to ensure that only valid model names are used when calling this
function. As per the current implementation, any string passed as the
`model_name` would be accepted.

The change enforces the function to only accept `gpt-4` or
`gpt-3.5-turbo` as valid model names. If any other model name is passed,
a ValueError is raised, informing the user about the invalid input and
suggesting the correct model names.

This change also adds tests to `tests/models/test_openai.py`. To ensure
an invalid model raises a value error.

Note: I added `gpt-4-1106-preview` in the tests, so when or if it's
added to the suite of models supported by outlines, that will need to be
updated, along with the mode in `openai.py`

Co-authored-by: Stephen Witkowski <[email protected]>

outlines/models/openai.py

@@ -104,6 +104,11 @@ def __init__(
             parameters that cannot be set by calling this class' methods.
 
         """
+        if model_name not in ["gpt-4", "gpt-3.5-turbo"]:
+            raise ValueError(
+                "Invalid model_name. It must be either 'gpt-4' or 'gpt-3.5-turbo'."
+            )
+
         try:
             import openai
         except ImportError:

tests/models/test_openai.py

@@ -1,6 +1,7 @@
 import pytest
 
 from outlines.models.openai import (
+    OpenAI,
     build_optimistic_mask,
     find_longest_intersection,
     find_response_choices_intersection,
@@ -48,3 +49,11 @@ def test_find_longest_common_prefix(response, choice, expected_prefix):
 def test_build_optimistic_mask(transposed, mask_size, expected_mask):
     mask = build_optimistic_mask(transposed, mask_size)
     assert mask == expected_mask
+
+
+def test_model_name_validation():
+    with pytest.raises(ValueError):
+        OpenAI(model_name="invalid_model_name")
+
+    with pytest.raises(ValueError):
+        OpenAI(model_name="gpt-4-1106-preview")