meta : keep the split for views inside one slab of the split axis
With a non-unified KV cache, build_attn_mha splits q into streams with a
view. For a permuted q, e.g. the absorbed query of glm5-next, that view
aborted with "view of permuted tensor not implemented". The split is now
kept if every other dim of the source and the view stays inside one slab
of the split axis. The per-device strides are only rescaled above it.
From tetherto/qvac-fabric-llm.cpp#353.
Co-authored-by: Pratik Narola <pratik.narola@tether.io>