[core] PlacementGroupSchedulingStrategy does not support placement_group=None

[krfricke]

What happened + What you expected to happen

The docstring says

    Attributes:
        placement_group: the placement group this actor belongs to,
            or None if it doesn't belong to any group.

but the implementation says:

        if placement_group is None:
            raise ValueError(
                "placement_group needs to be an instance of PlacementGroup"
            )

We should either adjust the docstring or allow None again.

It seems like passing scheduling_strategy="DEFAULT" achieves the same job, though this is not well documented.

Versions / Dependencies

Master, 2.0.0rc0

Reproduction script

from ray.util.scheduling_strategies import PlacementGroupSchedulingStrategy

PlacementGroupSchedulingStrategy(placement_group=None)

Issue Severity

Low: It annoys or frustrates me.

[rkooo567]

cc @jjyao this looks like a regression (or a bug when introducing this API). Can you confirm that?

[jjyao]

Thanks @krfricke for catching this!

@rkooo567 I thought it's working and it's already mentioned in the ray pg doc. But apparently this is not the case and code snippet in the doc is not tested by CI so we didn't catch this earlier. I'll fix this.