[Test] Fix bf16 rounding in selective_state_update reference

[sgurwinderr]

Purpose

selective_state_update_ref (the reference used by test_selective_state_update and test_mamba_ssm) computes out from a state that has already been rounded to state's dtype, diverging from the Triton kernel it validates against. This makes the reference wrong whenever state is narrower than C.

Root cause

The reference wrote the updated state back with state.copy_(...) — which rounds to state.dtype — before computing out = einsum(state.to(C.dtype), C). When state.dtype is narrower than C.dtype, out therefore sees a prematurely-rounded state. test_selective_state_update hits exactly this: with itype=torch.bfloat16, state is bf16 while C is created with no dtype (float32), so the reference's out is computed from a bf16-rounded state. The selective_state_update Triton kernel does not take that intermediate rounding step, so out and out_ref can diverge.

Fix

Compute out from the pre-rounding updated_state, then write state:

updated_state = state * dA + dB * rearrange(x, "b h d -> b h d 1")
out = torch.einsum("bhdn,bhn->bhd", updated_state.to(C.dtype), C)
state.copy_(updated_state)

In-place state semantics are preserved (state.copy_ still runs, so callers reading state afterward are unaffected). Only out changes, and only when state.dtype is narrower than C.dtype; for state.dtype == C.dtype it is a no-op.

Test plan

Verified on CPU that with state=bfloat16 / C=float32 the reorder changes out by ~0.09 (max abs) — the divergence test_selective_state_update's allclose(out, out_ref) checks — and is a bit-exact no-op when the dtypes match. Pure reference-function (test-only) change; no kernel/runtime code is touched.

Note: this mirrors an equivalent reference-ordering fix already validated on Intel XPU (intel/intel-xpu-backend-for-triton#7008 / #7013). The reference function has since been refactored from test_mamba_ssm.py into tests/kernels/mamba/utils.py, which is where this lands.

[claude]

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