Small speed improvements to --async-offload

[rattus128]

These two changes increase speed of bus-bound --async-offload. Tentative speedup is about 7% (although its hard to commit to a number on runpod where transfer speed fluctuates).

This is one of the better speedup measurements (QWEN FP16 512x512 on 5090):

Requested to load QwenImage
loaded partially; 22288.75 MB usable, 22278.42 MB loaded, 16689.48 MB offloaded, lowvram patches: 0
100%|███████████████████████████████████████████████| 10/10 [00:15<00:00,  1.59s/it]
Requested to load WanVAE
loaded completely; 267.67 MB usable, 242.03 MB loaded, full load: True
Prompt executed in 25.90 seconds
got prompt
loaded partially; 22250.75 MB usable, 22242.42 MB loaded, 16725.47 MB offloaded, lowvram patches: 0
100%|███████████████████████████████████████████████| 10/10 [00:15<00:00,  1.54s/it]
Requested to load WanVAE
loaded completely; 265.67 MB usable, 242.03 MB loaded, full load: True
Prompt executed in 19.76 seconds
got prompt
loaded partially; 22248.75 MB usable, 22242.42 MB loaded, 16725.47 MB offloaded, lowvram patches: 0
100%|███████████████████████████████████████████████| 10/10 [00:15<00:00,  1.55s/it]
Requested to load WanVAE
loaded completely; 265.67 MB usable, 242.03 MB loaded, full load: True
Prompt executed in 19.71 seconds

After

Requested to load QwenImage
loaded partially; 22298.75 MB usable, 22293.42 MB loaded, 16674.47 MB offloaded, lowvram patches: 0
100%|███████████████████████████████████████████████| 10/10 [00:14<00:00,  1.40s/it]
Requested to load WanVAE
loaded completely; 267.67 MB usable, 242.03 MB loaded, full load: True
Prompt executed in 119.17 seconds
loaded partially; 22248.75 MB usable, 22242.42 MB loaded, 16725.47 MB offloaded, lowvram patches: 0
100%|███████████████████████████████████████████████| 10/10 [00:13<00:00,  1.32s/it]
Requested to load WanVAE
loaded completely; 265.67 MB usable, 242.03 MB loaded, full load: True
Prompt executed in 17.32 seconds
loaded partially; 22248.75 MB usable, 22242.42 MB loaded, 16725.47 MB offloaded, lowvram patches: 0
100%|███████████████████████████████████████████████| 10/10 [00:13<00:00,  1.32s/it]
Requested to load WanVAE
loaded completely; 265.67 MB usable, 242.03 MB loaded, full load: True
Prompt executed in 17.13 seconds