context window wan vace use bug

### Custom Node Testing

- [x] I have tried disabling custom nodes and the issue persists (see [how to disable custom nodes](https://docs.comfy.org/troubleshooting/custom-node-issues#step-1%3A-test-with-all-custom-nodes-disabled) if you need help)

### Expected Behavior

Using the context window to handle the frame occupancy of the memory is the context length

### Actual Behavior

Using version 0.3.52, wan2.1 vace uses contenxt window, and found that the video memory occupancy has always been the occupation of the total length, shouldn't it be the occupation of each context_length, if so, it can directly generate a large length of frames, but the use of video memory is too large, what is the meaning of context window What problem does it solve long video generation? After testing, it was found that it can also be generated directly, with a length of more than 5 seconds, but the video memory occupies a lot of time

### Steps to Reproduce

### Debug Logs

```powershell
？？？？？
```

### Other

_No response_