torchao's Float8Tensor KernelPreference defaults to AUTO, which switches
the weight-quantize kernel to MSLK whenever an mslk package is importable
on sm90+. Measured on B200: that changes fp8 scale rounding bitwise (8/8
FLUX matrices differ, scales ~55 percent of bytes), so a box that merely
gains mslk would break the hosted-prequant bit-identity invariant; the
mslk path is also slower under torch.compile (opaque extern call blocks
inductor's quantize fusion, FLUX.1 fp8 e2e 1.149 to 1.624 s). Pin
KernelPreference.TORCH explicitly, matching current no-mslk behaviour
bit for bit; signature-gated for older torchao. GPU-smoked (finite,
rel err 0.037) and pinned by test.
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