Fix edge case in autoscaler with poor bin packing

[ericl]

Why are these changes needed?

When the cluster is backlogged, bump the "estimated busy nodes" count by the number of nonidle nodes + 1. This takes into account the case where the head node cannot accept tasks and hence is idle, or more generally, when nodes don't register as fully utilized due to poor bin packing, but there are tasks in a backlog somewhere in the cluster.

One side effect is that we are slightly more aggressive at scaling up, but this is probably ok.

Related issue number

Closes #5696

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://ray.readthedocs.io/en/latest/.

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[edoakes]

Fixing some of the unnecessary output too, nice :)

LGTM assuming you've validated manually.

[edoakes]

This seems very aggressive, for example in a case where we somehow have nearly the whole cluster idle but a single node with a queue (could happen if there were many short-lived tasks scheduled on other nodes?). Maybe we should make it a bit more of a heuristic instead of this hard rule. Seems like a fine fix for now, though.

[ericl]

This won't trigger an up-scaling event unless there is at least "some" load on almost every node. So the case you mentioned is probably OK.

But yeah agreed we could be smarter, perhaps by looking at the queue size and extrapolating the load.