prefetch: guard against no offload (CORE-149)

[rattus128]

(Urgent) crasher fix for LTX2.3 on RTX5090 or greater.

Example test conditions:

RTX5090, linux LTX2.3 FP8

Before:

  File "/home/rattus/ComfyUI/comfy/ldm/lightricks/av_model.py", line 1067, in forward
    return super().forward(
           ^^^^^^^^^^^^^^^^
  File "/home/rattus/ComfyUI/comfy/ldm/lightricks/model.py", line 879, in forward
    return comfy.patcher_extension.WrapperExecutor.new_class_executor(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/rattus/ComfyUI/comfy/patcher_extension.py", line 112, in execute
    return self.original(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/rattus/ComfyUI/comfy/ldm/lightricks/model.py", line 932, in _forward
    x = self._process_transformer_blocks(
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/rattus/ComfyUI/comfy/ldm/lightricks/av_model.py", line 915, in _process_transformer_blocks
    comfy.model_prefetch.prefetch_queue_pop(prefetch_queue, vx.device, block)
  File "/home/rattus/ComfyUI/comfy/model_prefetch.py", line 40, in prefetch_queue_pop
    offload_stream.wait_stream(comfy.model_management.current_stream(device))
    ^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'wait_stream'

After:

Model VideoVAE prepared for dynamic VRAM loading. 1384MB Staged. 0 patches attached.
Prompt executed in 23.79 seconds

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📥 Commits

Reviewing files that changed from the base of the PR and between f3ea976 and 85d7141.

📒 Files selected for processing (1)

• comfy/model_prefetch.py

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📝 Walkthrough

Walkthrough

The pull request modifies prefetch_queue_pop() in comfy/model_prefetch.py to conditionally execute the offload_stream.wait_stream() call only when offload_stream is not None. Previously, this call was made unconditionally after consuming a queue entry. The guard prevents the function from attempting to wait on a missing stream. The remainder of the consumed-entry cleanup flow, including unpacking prefetch_state and conditionally calling cleanup_prefetched_modules(), remains unchanged.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'prefetch: guard against no offload' directly describes the main change: adding a guard to handle cases where offload_stream is None, which is exactly what the changeset accomplishes.
Description check	✅ Passed	The description is directly related to the changeset, providing context about the bug (AttributeError when offload_stream is None), reproducing conditions, and the fix's effectiveness.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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