[MoE] Preserve TP sharding with explicit EP

[LiuYinfeng01]

Summary

• preserve physical TP size/rank inside experts when explicit EP is configured
• use the fixed-lane EP group for expert mapping and pre-I/O weight filtering
• keep AG/RS dispatch/combine over EP and the existing late TP all-reduce
• add coverage for TP4/EP2 config, Kimi expert ownership, loader filtering, final TP reduction, and the ROCm latent tail

Stacked on #52099. The topology commit disappears once #52099 merges.

Kimi-K3 has 896 routed experts. With EP2, each process owns 448 experts with TP4-sharded weights; four ranks in a TP group own distinct shards of the same 448-expert set. No Kimi production-code special case is required: the existing ROCm latent runner consumes the preserved physical TP size.

8-GPU validation

• topology: TP4/DP2/EP2, AG/RS, target-only, 100K input + 1K output, concurrency 1
• startup log: Local/global number of experts: 448/896; weight filter loads 448/896
• execution log: MoEPrepareAndFinalizeNaiveDPEPModular + AiterExperts
• numerical gate: GSM8K 100/100 strict, 100/100 flexible, zero errors/malformed
• TPOT over 3 runs: 27.296 ms mean (27.107–27.447 ms); correctness passes, but this topology is not the C1 performance winner
• non-MTP performance gate is met by TP8/DP1: 18.059 ms mean TPOT over 3 runs (18.056–18.062 ms)

Test plan

• topology + generic execution suite — 13 passed
• Kimi ROCm latent runner + hybrid MoE suite — 19 passed on 8 GPUs
• Ruff check and format check on changed files
• git diff --check
• 8-GPU TP4/DP2/EP2 Kimi-K3 serving and GSM8K validation

[claude]

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