afmoe ring attention support.

[sapiosaturn]

tested on an RL run, reward goes up, eval goes up, mismatch looks good. however, not a huge fan of how it's currently a special case for the afmoe model, but unsure how this could be streamlined (with gated attn and all).

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Note

Medium Risk
Touches core attention execution and distributed CP ring-attention patching, so regressions could affect training correctness/perf across models (especially sliding-window/FA3 paths).

Overview
Enables context-parallel ring attention for AFMoE by refactoring FlashAttention/AfmoeFlashAttention to route kernel invocation through a _compute_attention hook and patching that hook to a ring-attention implementation via the new substitute_ring_attn.

Extends the FA3 ring-attention wrapper (ring_fa3_varlen_func) to accept and forward window_size, so sliding-window/local-attention layers and full-attention layers work correctly under ring attention. RL and SFT training now call substitute_ring_attn when cp is enabled, and the changelog documents the new AFMoE+CP support.

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