The previous gate (len(cond_or_uncond) == 2 and set == {0, 1}) was
intended to skip the cond/uncond swap when only one half was present
under MultiGPU CFG Split, but it was too restrictive: it also skipped
batch_size > 1 + CFG (cond_or_uncond like [0, 0, 1, 1] or [0,0,0,0,
1,1,1,1]), where chunk(2) still splits the batch cleanly into a cond
half and an uncond half and the swap is still required.
Switch to context.shape[0] >= 2, matching the parallel fix landed on
master in #13699. The swap is a permutation-invariant no-op when the
two halves don't form a CFG pair (since the output swap_cfg_halves
block immediately undoes the permutation), so the only thing the gate
actually needs to do is guard against chunk(2) on a batch of one.
Amp-Thread-ID: https://ampcode.com/threads/T-019e4a00-fe3d-76bd-a2f2-a8c8c4040082
Co-authored-by: Amp <amp@ampcode.com>
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