Comment-only pass over the Python this PR touches: drop what the code already
says, collapse multi-line explanations that still read on one line, and keep
the reasoning that is not recoverable from the code. No code, docstring
semantics or behaviour changes; verified with an AST comparison against the
previous revision, and the backend suite is unchanged (same 37 environment
failures as before: the API integration tests that need a live keyed server,
the flash-attn install hooks, and the GPU memory fields).
The image bench drove pipe() directly with no cache reset between the
warmup and measured prompts, while the production backend clears the
FBCache residuals before each generation. diffusers keys those residuals
on the long-lived transformer and never resets them itself, so step 0 of
each measured prompt compared its first-block residual against the
previous prompt's final one, a state production never runs. Mirror the
backend's _reset_step_cache (best-effort, no-op for uncached configs and
for SDXL's unet) inside _generate so both warmup and measured passes
start clean, and note that pre-fix FBCache rows may overstate results.
Applies the video round-2 accuracy findings to the image diffusion stack and fixes
two real image-path bugs found while measuring. All numbers B200, production
settings (family default steps/guidance, 1024px, seed 42, 4 fixed prompts), LPIPS
(AlexNet) via the new scripts/image_speedmem_bench.py, which drives the production
lever functions in the loader's own order.
- inductor precision parity: emulate_precision_casts=True on the regional-compile
path (fused pointwise kernels keep fp32 intermediates where eager rounds to bf16
between ops). Pairwise LPIPS of the compiled tier vs the same-stack eager tier:
Qwen-Image 0.019 to 0.006 at identical speed (72.4 vs 72.5 ms/step), FLUX.1-dev
0.046 to 0.029 at +2% step time (69.8 vs 68.3, reproduced), FLUX.2-klein-4B
0.018 to 0.017 at identical speed. Snapshot/restored with the other process-wide
backend flags so an off load never inherits it.
- cache x compile composition: re-point each cache hook's fn_ref.original_forward
at a torch.compile'd wrapper of the same bound method (armed only where the
speed layer compiled the block; restored before every disable_cache and before
the partial-hook cleanup). Qwen-Image FBCache computed steps 91.8 to 71.2 ms
(back at the uncached compiled rate), 1.21x end to end (7.36 to 6.06 s per 4
images); FLUX.1-dev already traced through its FBCache hook and is measured
neutral (same-process armed vs unarmed latents bit-identical). Skip counts
within noise (13 vs 11 of 76; pairwise LPIPS 0.005).
- FBCache mid-session toggle crash: diffusers 0.39 caches the HookRegistry child
list on first cache_context use, so an uncached generation followed by a
20+-step generation (the auto toggle path) enabled hooks the context never
reached and crashed with "No context is set" (reproduced live on FLUX.1-dev).
Invalidate the stale child cache after every enable_cache.
- TE fp8_dynamic zero-row guard: torchao per-row fp8 derives a per-output-channel
scale from the row amax, so an all-zero weight row is 0/0 = NaN. SDXL's
text_encoder_2 (OpenCLIP bigG) ships exactly such a row, and every explicit
fp8_dynamic SDXL render came out black; keep zero-row Linears dense (LPIPS
0.976 black to 0.096 working). Other families' encoders have no such rows and
are byte-identical.
- No AUTO TE quant exists on the image branch (text_encoder_quant defaults dense,
explicit-only), so the video round's auto-dense retune has no image analogue;
the explicit lever's cost is now measured (TE fp8_dynamic alone, LPIPS vs
bit-exact: Qwen-Image 0.038, FLUX.1-dev 0.084, SDXL 0.096; no speed win, VRAM
-6.5 GB on Qwen-Image) for the docs.
Tests: 96 passing across the cache/speed/precision suites (11 new arming, 2
child-registry, 2 zero-row, 4 inductor-flag); ruff clean.
