fix mhc device

[valarLip]

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[github-actions]

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[Copilot]

Pull request overview

Fixes mhc_pre intermediate tensor allocations to be created on the same device as the input residual, preventing device-mismatch failures when the global default device is not CUDA.

Changes:

• Reorders imports in aiter/ops/mhc.py.
• Allocates out_pad, sqrsum, post_mix, comb_mix, and layer_input with device=residual.device.

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[Copilot]

Consider adding a regression test that exercises mhc_pre when the global default device is CPU but inputs are explicitly on CUDA (e.g., torch.set_default_device('cpu') then create residual/fn/hc_scale/hc_base on cuda). This change fixes internal tensor allocations to follow residual.device, but current tests may still pass even if allocations accidentally fall back to the default device.