[autoscaler]: Assertion in info string can kill autoscaler

[ls-daniel]

In python/ray/autoscaler/autoscaler.py there's an info string that gets logged occasionally.

The line assert used >= 0 is present here. I don't think that assertions should be happening in an informational string.

I can't easily reproduce this but I occasionally have this causing issues (if it blows up during an autoscaler.update call more than a few times, eventually the autoscaler will just bomb itself 'Too many errors, abort').

Can I just remove the assertion?

[ericl]

Yeah, we should set used = max(0, used) instead to guard against this.