fix: make store-based barrier reusable across updates

[RunningLeon]

fix: make the shared-store barrier reusable across updates in #106

Summary

• add an independent generation counter for ParameterServer.store_based_barrier();
• use a unique _store_based_barrier() group name for every invocation;
• keep the long-lived root TCPStore unchanged and shared;
• add CPU-only regression coverage for two consecutive barriers on the same TCPStore.

[koriyoshi2041]

Verified the repeated-barrier path on the exact head. The generation counter is independent of process-group creation, every rank deterministically advances it once per successful update barrier (including auto_pg=False), and the shared-store regression exercises two consecutive generations. Locally, tests/test_store_barrier.py passes 2/2; focused Ruff check/format and diff hygiene also pass.

[RunningLeon]

@koriyoshi2041 hi, is there a plan to merge this PR?

[koriyoshi2041]

I reviewed this as an external contributor and don't have merge rights in this repository, so I can't give a maintainer merge timeline. The PR is still at the exact head I approved (300f5c2), both hosted checks are green, and I found no correctness blocker in the repeated shared-store barrier path. A repository maintainer will need to make the merge decision.

[koriyoshi2041]

Re-reviewed the requested key cleanup at exact head c63195f. Rank 0 only deletes generation N after completing N+2; completion of N+2 means every rank has entered that later barrier, so no rank can still depend on N's counter or last_worker key. Cleanup failure remains non-fatal and bounded to a warning.

Locally, the real shared-TCPStore regression passes all 10 generations and verifies generations 1–8 are removed while 9–10 remain (2/2 focused tests). Ruff check and format also pass on both changed files. Approving the updated head.