[Distributed][HCCL] Harden all_to_all_single split validation and add 2/4/8-card tests

[lvyufeng]

Summary

• Harden HCCL all_to_all_single split validation to use store-synchronized call sequencing and full matrix consistency checks before dispatch.
• Ensure invalid split pairing fails at Python validation stage on all ranks, avoiding partial-rank fallthrough into HCCL recv/send errors.
• Expand HCCL all_to_all_single multicard coverage to 2/4/8 for both async unequal semantics and invalid split validation, with transient init retry handling.

Key Changes

• src/mindtorch_v2/distributed/__init__.py
• tests/mindtorch_v2/test_hccl_all_to_all_single_async_unequal_multicard.py
• tests/mindtorch_v2/test_hccl_all_to_all_single_invalid_splits_multicard.py

Test Plan

• PYTHONPATH=/usr/local/Ascend/ascend-toolkit/latest/python/site-packages:src pytest -q tests/mindtorch_v2/test_hccl_all_to_all_single_invalid_splits_multicard.py
• PYTHONPATH=/usr/local/Ascend/ascend-toolkit/latest/python/site-packages:src pytest -q tests/mindtorch_v2/test_hccl_all_to_all_single_async_unequal_multicard.py
• PYTHONPATH=/usr/local/Ascend/ascend-toolkit/latest/python/site-packages:src python tests/mindtorch_v2/test_hccl_all_to_all_multicard.py
• PYTHONPATH=/usr/local/Ascend/ascend-toolkit/latest/python/site-packages:src pytest -q tests/mindtorch_v2/test_hccl_mvp_multicard.py
• PYTHONPATH=src pytest -q tests/mindtorch_v2/contract/test_schema_registration_order.py tests/mindtorch_v2/contract/test_schema_coverage.py

Notes

• CI environments without Ascend devices may not execute the HCCL multicard paths; these checks were validated on local Ascend hardware.