fix(ds4): use monolithic backend for single-device launches, not only for HIP

[Graffioh]

Summary

• route DeepSeek 4 single-device launches through DeepSeek4Backend on both CUDA and HIP
• reserve LayerSplitBackend for explicit multi-device placement or a configured remote target shard
• preserve automatic CUDA/Halo layer splitting whenever remote target-shard IPC is enabled

Why

CUDA launches previously always selected DeepSeek4LayerSplitAdapter. Without an explicit or remote split, its automatic sizing capped the local shard at 42 layers, so a single H200 executed a virtual [0, 42) + [42, 43) split on the same GPU.

That virtual boundary adds split orchestration overhead and prevents the full-model fused decode path, whose eligibility requires the same range to start at layer 0 and own the output layer. A single local device should instead use the monolithic backend; devices that cannot fit the full model can use its existing hybrid-expert fallback.

Expected behavior

Configuration	Backend	Behavior
H200/Halo single	DeepSeek4Backend	Full monolithic
3090 single	DeepSeek4Backend	Full attempt → hybrid fallback
3090 + Halo IPC	LayerSplitBackend	Auto-split CUDA/Halo
Multi-GPU explicit	LayerSplitBackend	Layer split

Validation

• git diff --check
• Run on H200 both single and virtual split (based on fix(ds4): correct layer-split HC handoff, per-device state, and greedy decode in local path #507)

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}