VertexAI now allows to tune codey models (#10367)

Description: VertexAI now supports to tune codey models, I adapted the
Vertex AI LLM wrapper accordingly
https://cloud.google.com/vertex-ai/docs/generative-ai/models/tune-code-models

libs/langchain/langchain/llms/vertexai.py

@@ -169,7 +169,7 @@ class VertexAI(_VertexAICommon, LLM):
         tuned_model_name = values.get("tuned_model_name")
         model_name = values["model_name"]
         try:
-            if tuned_model_name or not is_codey_model(model_name):
+            if not is_codey_model(model_name):
                 from vertexai.preview.language_models import TextGenerationModel
 
                 if tuned_model_name:
@@ -181,7 +181,12 @@ class VertexAI(_VertexAICommon, LLM):
             else:
                 from vertexai.preview.language_models import CodeGenerationModel
 
-                values["client"] = CodeGenerationModel.from_pretrained(model_name)
+                if tuned_model_name:
+                    values["client"] = CodeGenerationModel.get_tuned_model(
+                        tuned_model_name
+                    )
+                else:
+                    values["client"] = CodeGenerationModel.from_pretrained(model_name)
         except ImportError:
             raise_vertex_import_error()
         return values