[Qwen3.5] Fix MoE expert weight layouts in state dict adapter

[taking-lying-flat]

Summary

• align Qwen3.5 MoE grouped expert conversion with the current Transformers state-dict layouts
• split and fuse gate_up_proj along the intermediate dimension without transposing
• preserve the shared [E, D, F] layout for HF down_proj and TorchTitan w2_EDF

Transformers stores gate_up_proj as [E, 2F, D] and down_proj as [E, D, F]. TorchTitan stores w1_EFD and w3_EFD as [E, F, D], and w2_EDF as [E, D, F].

Testing

• pre-commit formatting, AST, flake8, pydoclint, codespell, and repository hygiene hooks
• targeted Pyrefly check: 0 errors
• shape/value round-trip with asymmetric expert dimensions
• asymmetric expert forward parity between HF and TorchTitan layouts