fix: Add PipelineVariable support to ModelTrainer fields (fixes #5524)

Extend StrPipeVar type to ModelTrainer's direct fields:
- training_image: Optional[str] -> Optional[StrPipeVar]
- algorithm_name: Optional[str] -> Optional[StrPipeVar]
- training_input_mode: Optional[str] -> Optional[StrPipeVar]
- environment: Dict[str, str] -> Dict[str, StrPipeVar]

This follows the existing V3 pattern already used by SourceCode,
OutputDataConfig, and Compute (for instance_type). The StrPipeVar
type alias and PipelineVariable.__get_pydantic_core_schema__()
already exist in the codebase.

This unblocks V2->V3 migration for SageMaker Pipelines users who
need to pass ParameterString to ModelTrainer fields.

Fixes #5524

Author: Amit Modi

sagemaker-train/src/sagemaker/train/model_trainer.py

@@ -116,6 +116,7 @@
 from sagemaker.core.jumpstart.utils import get_eula_url
 from sagemaker.train.defaults import TrainDefaults, JumpStartTrainDefaults
 from sagemaker.core.workflow.pipeline_context import PipelineSession, runnable_by_pipeline
+from sagemaker.core.helper.pipeline_variable import StrPipeVar
 
 from sagemaker.train.local.local_container import _LocalContainer
 
@@ -235,14 +236,14 @@ class ModelTrainer(BaseModel):
     compute: Optional[Compute] = None
     networking: Optional[Networking] = None
     stopping_condition: Optional[StoppingCondition] = None
-    training_image: Optional[str] = None
+    training_image: Optional[StrPipeVar] = None
     training_image_config: Optional[TrainingImageConfig] = None
-    algorithm_name: Optional[str] = None
+    algorithm_name: Optional[StrPipeVar] = None
     output_data_config: Optional[shapes.OutputDataConfig] = None
     input_data_config: Optional[List[Union[Channel, InputData]]] = None
     checkpoint_config: Optional[shapes.CheckpointConfig] = None
-    training_input_mode: Optional[str] = "File"
-    environment: Optional[Dict[str, str]] = {}
+    training_input_mode: Optional[StrPipeVar] = "File"
+    environment: Optional[Dict[str, StrPipeVar]] = {}
     hyperparameters: Optional[Union[Dict[str, Any], str]] = {}
     tags: Optional[List[Tag]] = None
     local_container_root: Optional[str] = os.getcwd()