Fix FA3 ring attention argument forwarding

[zjn20030811]

Summary

• fix the FlashAttention-3 ring wrapper call so optional descaling values are passed through the custom autograd function using positional arguments
• keep compatibility with yunchang FA3 adapters that do not expose descaling keyword parameters when the values are unset
• return the correct number of gradients for the expanded autograd input contract

This addresses #616, where xFuserLongContextAttention could fail before entering the FA3 kernel because autograd.Function.apply does not accept keyword arguments.

Validation

• python -m pytest -q tests/core/test_ring_flash_attn_fa3_args.py (4 passed)
• python -m pytest -q tests/core/test_ring_flash_attn_fa3_args.py tests/core/test_ring_flash_attn.py (4 passed, 1 skipped because flash-attn is unavailable)
• python -m compileall -q xfuser/core/long_ctx_attention/ring/ring_flash_attn.py tests/core/test_ring_flash_attn_fa3_args.py
• git diff --check