Move all chunked prefill attention computation from attention.py to
SparsePolicy.compute_chunked_attention(). This is the v4 architecture
refactoring for sparse attention policies.
Changes:
- Add compute_chunked_attention abstract method to SparsePolicy base
- Add offload_engine parameter to select_blocks for policies needing
KV access during block selection
- Implement compute_chunked_attention in FullAttentionPolicy with
complete ring buffer pipeline logic
- Simplify attention.py to delegate all chunked prefill to policy
- Remove redundant _sync_load_previous_chunks and
_ring_buffer_pipeline_load methods from Attention class
Test: test_needle.py --enable-offload PASSED
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
