ciflow/torchtitan/184819: [DTensor] Fix group_norm scalar adjuster crash when weight=None

The _adjust_group_norm_scalars function counts DTensorSpec args to find

where the N/C/HxW scalar args start. When weight or bias is None (e.g.

GroupNorm with affine=False), the None slots are not DTensorSpec, so the

count is too low and the adjuster overwrites the None weight slot with

the local N value. This causes a RuntimeError: expected Optional[Tensor]

but got int.

Fix by counting None slots alongside DTensorSpec when computing the

offset. Adds a regression test for group_norm with weight=None.

Fixes #184816

Authored by Claude.