An i2v generation died in the first denoising step with a bare AssertionError out
of flash_attn_bsa_3d, on a geometry the guard had just declared valid: 93 frames,
sides divisible by 64.

bsa_problems only ever checked the noise branch. A conditioning pass does not tile
the latent once — Attention.forward splits the sequence into a conditioning block
and a noise block and calls flash_attn_bsa_3d on each (modules/attention.py:123-134),
so the cond depth and the remaining noise depth must divide by the chunk as well.
generate_i2v hardcodes one conditioning latent (pipeline:796) and 1 % 4 != 0, which
means block-sparse attention has never once been able to run an i2v pass, at any
frame count or resolution. The avatar pipeline hardcodes the same 1 in four places,
and generate_vc computes a count it never rounds. Only generate_refine gets it
right, by padding both counts to the granularity itself (pipeline:1245-1250).

(a) The guard now knows about the split. _cond_latent_passes enumerates every
conditioning count the request can produce — segment 0 uses the task's own entry
point and every later segment uses generate_vc, so even t2v runs a conditioning
pass once num_segments > 1, and BSA is one flag for the whole request — and a
count that cannot tile puts the pass on dense attention with a line saying which
condition failed.

(b) bsa_pad_cond, OFF by default, rounds the count up to the granularity on the way
into the DiT, which is what generate_refine does for itself, so an i2v or
continuation pass can keep BSA. Patched onto the DiT instance rather than the
vendored source, the same way the denoising bars are rebound: the count is born
inside generate_i2v's loop, so there is no outer argument to correct. Off by
default because the pipeline marks only the real conditioning frames clean
(timestep[:, :1] = 0), so rounding 1 up to 4 hands the conditioning block three
latent frames that are still noise — well-defined attention, but not what the model
was trained on, and on a video model that kind of error shows up as temporal drift.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>