Move existing PPR sampler work off event loop

[mkolodner-sc]

Summary

Moves the existing distributed PPR sampler's CPU-heavy C++ work off the asyncio event-loop path.

This PR is limited to the existing/non-typed PPR path. Typed-channel PPR enablement is left for the next PR in the stack.

Stack

• Base: Clean up PPR sampler naming #707
• Next: Add typed-channel PPR sampler #709

Reasoning for Change

PPR sampling runs several CPU-heavy C++ steps while async graph-store/RPC work is also in flight. When those steps run on the event-loop thread or hold the GIL longer than necessary, they can delay RPC completion callbacks and reduce concurrency across sampler coroutines.

This change keeps the PPR math and returned sequences unchanged while moving construction, queue draining, residual pushes, and extraction through executor/GIL-release boundaries where possible.

Changes

• Releases the GIL around PPRForwardPush construction, queue draining, residual push, and extraction in the pybind layer.
• Runs PPR state construction, drain_queue, push_residuals, and top-k extraction through run_in_executor from the Python sampler.
• Removes a redundant frontier .to(device) before one-hop fetches; extracted tensors are still moved to the sampler device at the existing output boundary.

[kmontemayor2-sc]

Thanks Matt!

[kmontemayor2-sc]

LGTM! Nice work Matt :)

Do you think there's similar low-hanging (but hard to find!) fruit here for the khop sampler?

E.g. is it possible to put this bit in the background? https://github.com/alibaba/graphlearn-for-pytorch/blob/main/graphlearn_torch/python/distributed/dist_neighbor_sampler.py#L297

Or do we need to also write a CPP version of that _sample_from_nodes so we can force-detach the gil? May not be difficult? WDYT?