## Motivation
Support cancelling image generation, similar to existing support for
cancelling text generation
## Changes
- Dashboard (app.svelte.ts): Wire up AbortController for both
generateImage and editImage API calls. On abort, show "Cancelled"
instead of an error. Clean up the controller in finally.
- Pipeline runner (pipeline/runner.py): Introduce a cancel_checker
callback and NaN-sentinel cancellation protocol for distributed
diffusion:
- _check_cancellation() - only rank 0 polls the cancel callback
- _send() - replaces data with NaN sentinels when cancelling, so
downstream ranks detect cancellation via _recv_and_check()
- _recv() / _recv_like() wrappers that eval and check for NaN sentinel
- After cancellation, drains any pending ring recv to prevent deadlock
- Skips partial image yields and final decode when cancelled
- Image runner (runner/image_models/runner.py): Deduplicate the
ImageGeneration and ImageEdits match arms into a shared
_run_image_task() method. Thread a cancel_checker closure (backed by the
existing cancel_receiver + cancelled_tasks set) into generate_image().
- Plumbing (distributed_model.py, generate.py): Pass cancel_checker
through the call chain.
## Why It Works
- Rank 0 is the only node that knows about task-level cancellation. When
it detects cancellation, it sends NaN tensors instead of real data.
Higher-order ranks detect the NaN sentinel on recv, set their own
_cancelling flag, and propagate NaN forward
- A drain step after the loop prevents the deadlock case where the last
rank already sent patches that the first would never consume.
- For single-node mode, the loop simply breaks immediately on
cancellation.
## Test Plan
### Automated Testing
New tests in src/exo/worker/tests/unittests/test_image
ciaranbor
6de14cfedb
