Fix torchrun injection: add ancestor-step label, use torchrun command

- Add trainer.kubeflow.org/trainjob-ancestor-step label to replicatedJob
  template so Trainer controller injects PET_* env vars
- Change TrainJob command from python to torchrun so PET_* env vars
  are used for distributed training setup
- Remove command from Runtime (keep it in TrainJob only)

Verified: 2-node 16-GPU distributed training running successfully

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

configs/volcano-trainjob-integration.yaml

@@ -310,6 +310,9 @@ spec:
       replicatedJobs:
       - name: node
         template:
+          metadata:
+            labels:
+              trainer.kubeflow.org/trainjob-ancestor-step: trainer
           spec:
             template:
               metadata:
@@ -376,7 +379,7 @@ spec:
   trainer:
     image: nvcr.io/nvidia/pytorch:24.10-py3
     command:
-      - python
+      - torchrun
       - /workspace/scripts/train_nccl.py
     numNodes: 2
     resourcesPerNode: