KL Consistency Tests

What the test is for

kl_test_utils scores the same token twice -- once as a prefill input logprob, once as a decode output logprob -- and compares. The two paths run different kernels over different shapes, so agreement is a statement about state, not about answer quality: it catches a radix-cache prefix that does not reproduce a fresh prefill, a stale conv/mamba checkpoint, a SWA pool that evicted something it still needed.

gsm8k passing says nothing about this. Accuracy is insensitive to a handful of corrupted tokens; the KL check is not.

Two independent conditions produce a zero

Reaching bit-identity needs both, and they fail for unrelated reasons. Knowing which one a nonzero belongs to is most of the debugging.