[Autoscaler][V2] Use running node instances to rate-limit upscaling

[ryanaoleary]

Why are these changes needed?

This PR changes the v2 autoscaler scale up logic to use IMInstance.RAY_RUNNING (rather than IMInstance.REQUESTED + nodes with an IMInstance.cloud_instance_id) when computing the number of nodes to launch per node type. This is to maintain consistency with how upscaling_speed is handled by the V1 autoscaler and described in the Ray docs, i.e. the factor by which to scale the number of nodes in the cluster, with a minimum of 5.

Related issue number

Closes #50259

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    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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