## Motivation
There's no way to easily use the cancellation features we added! Also,
prefill can take ages so let's allow cancelling out of that.
## Changes
Wiring up our existing functionality to easily cancel during generation
(and adding stuff to do so during prefill)
## Test Plan
### Manual Testing
Tested it works during both prefill and decode.
### Automated testing
Needs testing to see if this causes a GPU timeout error on large prefill
on large models in pipeline parallel. However, from manually testing GLM
5 pipeline ring on 2 nodes, and from reading the code, it does not seem
like this will be the case.
