SparseGroupNorm / SparseLayerNorm used torch.zeros_like which fails with
"DispatchStub: missing kernel for mps" on PyTorch builds compiled with
both CUDA and MPS backends (the user's local build hit this). Added a
_zeros_like_safe helper that builds zeros on CPU and transfers when the
reference tensor is on MPS; Apple Silicon unified memory makes the
transfer metadata-only, so the overhead vs a working MPS zeros kernel
is negligible. On CPU, behaves identically to torch.zeros_like.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
