chore: LLM - Change tuning to use "@" instead of "-" as the model version delimiter

The "<model>-<version>" format was only supported for text-bison-001 and will be deprecated in the future.

PiperOrigin-RevId: 550710642

Author: Ark-kun

tests/unit/aiplatform/test_language_models.py

@@ -672,6 +672,7 @@ def test_tune_model(
                 "pipeline_job"
             ].runtime_config.parameter_values
             assert pipeline_arguments["learning_rate"] == 0.1
+            assert pipeline_arguments["large_model_reference"] == "text-bison@001"
             assert (
                 call_kwargs["pipeline_job"].encryption_spec.kms_key_name
                 == _TEST_ENCRYPTION_KEY_NAME

vertexai/_model_garden/_model_garden_models.py

@@ -117,7 +117,7 @@ def _get_model_info(
 
     if short_model_id in _SHORT_MODEL_ID_TO_TUNING_PIPELINE_MAP:
         tuning_pipeline_uri = _SHORT_MODEL_ID_TO_TUNING_PIPELINE_MAP[short_model_id]
-        tuning_model_id = short_model_id + "-" + publisher_model_res.version_id
+        tuning_model_id = publisher_model_template.rsplit("/", 1)[-1]
     else:
         tuning_pipeline_uri = None
         tuning_model_id = None