fix vertexai codey models (#12173)

**Description:**

This PR fixes issue #12156 by checking for Codey models appropriately
before result parsing.


Maintainer: @hwchase17 , @agola11

libs/langchain/langchain/llms/vertexai.py

@@ -299,7 +299,12 @@ class VertexAI(_VertexAICommon, BaseLLM):
                 res = completion_with_retry(
                     self, prompt, run_manager=run_manager, **params
                 )
-                generations.append([_response_to_generation(r) for r in res.candidates])
+                if self.is_codey_model:
+                    generations.append([_response_to_generation(res)])
+                else:
+                    generations.append(
+                        [_response_to_generation(r) for r in res.candidates]
+                    )
         return LLMResult(generations=generations)
 
     async def _agenerate(

libs/langchain/tests/integration_tests/llms/test_vertexai.py

@@ -36,6 +36,15 @@ def test_vertex_generate() -> None:
     assert len(output.generations[0]) == 2
 
 
+@pytest.mark.scheduled
+def test_vertex_generate_code() -> None:
+    llm = VertexAI(temperature=0.3, n=2, model_name="code-bison@001")
+    output = llm.generate(["generate a python method that says foo:"])
+    assert isinstance(output, LLMResult)
+    assert len(output.generations) == 1
+    assert len(output.generations[0]) == 2
+
+
 @pytest.mark.scheduled
 @pytest.mark.asyncio
 async def test_vertex_agenerate() -> None: