Fix(pw): correct atom index in non-collinear non-local stress on GPU

[dyzheng]

In the nspin=4 cal_stress_nl CUDA/ROCm kernels, deeq_nc was indexed with iat + ia while iat was also incremented inside the atom loop, so the atom offset was counted twice. For element types with more than one atom (e.g. Fe2 or HEAs), the second atom onwards read deeq_nc at wrong indices (aliasing another npol block, or out of bounds for the ps3 term), producing a wrong non-local stress with spurious off-diagonal components.

Use iat alone, consistent with the collinear stress kernel and the non-collinear force kernel, in both the CUDA and ROCm kernels.

Add CPU/GPU parity tests for the deeq_nc (nspin=4) overload of cal_stress_nl_op with ntype=1 and atom_na={2}, which fail on the buggy GPU kernel and pass after the fix. Existing tests only covered the real/collinear overload and could not catch this.

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[mohanchen]

Thanks.