- Add training_worker.py: long-lived subprocess that handles GPU training
work, owns HF model wrapper (views into vLLM GPU memory), Apollo
optimizer, and checkpoint sync
- train_router.py: now forwards /train requests via async ZMQ instead of
running training in-process. Adds /checkpoint and /train/status endpoints
- export_hook.py: store model_path in __metadata__ so training worker can
find it without cross-process communication
- This fixes two bugs:
1. Process boundary issue - model_path was set in worker process but
needed in API server process
2. Blocking event loop - training blocked vLLM's async event loop
Architecture: vLLM API server <-> ZMQ <-> training subprocess
The subprocess loads IPC handles once, creates views into vLLM's GPU
memory, and handles training requests without blocking inference.
Co-Authored-By: Proof of Concept <poc@bcachefs.org>
