[Core] Composable parallel: Phase 1 strategy overlay (TP/DP/stage-replica) (RFC #4084) (#4281)

Signed-off-by: Kushan A. <kahmadian@nvidia.com>
Signed-off-by: kushanam <kushanam@users.noreply.github.com>
Signed-off-by: Kushan A <kahmadian@nvidia.com>
Signed-off-by: kahmadian <kahmadian@users.noreply.github.com>
Co-authored-by: kushanam <kushanam@users.noreply.github.com>
Co-authored-by: kahmadian <kahmadian@users.noreply.github.com>

vllm_omni/engine/arg_utils.py

@@ -434,6 +434,9 @@ class OrchestratorArgs:
     stage_configs_path: str | None = None
     deploy_config: str | None = None
     stage_overrides: str | None = None  # raw JSON string; parsed downstream
+    # Optional composable-parallel strategy.yaml; orchestrator reads it, overlays
+    # derived sizing onto merged stages, then drops it before per-stage engine args.
+    strategy_config: str | None = None
 
     # === Mode Switches (orchestrator reads, DeployConfig redistributes) ===
     async_chunk: bool | None = None