feat: support ring attention with arbitrary KV sequence lengths (#13545)
* feat: support ring attention with arbitrary KV sequence lengths
* fix: align ring_anything with ulysses_anything (size gather + unshard)
* docs: document ring_anything mode
* fix: merge hook branches, add ring_anything comment + guard
* docs: address ring_anything review comments
* docs: update ring_anything guidance
* docs: refine ring_anything guidance per review
* fix: address ring_anything style check
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Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>