[Autoscaler][Placement Group] Skip placed bundle when requesting resource

[mimiliaogo]

Why are these changes needed?

Before the PR, when a node in a placement group (PG) goes down, the autoscaler attempts to reschedule the entire PG (all bundles). However, this will lead to overprovisioning. Details: #40212

This PR solved this by skipping already placed bundles (i.e., bundles with an associated node_id) when demanding resources in autoscaler.

Before: Every bundles get rescheduled

After: Only one node will be scaled up

Related issue number

Closes #40212

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(