Move decode attention computation from attention.py to SparsePolicy:
- Add compute_chunked_decode abstract method to SparsePolicy base class
- Implement compute_chunked_decode in FullAttentionPolicy with:
- Ring buffer pipeline (_decode_ring_buffer_pipeline)
- Cross-layer pipeline (_decode_with_layer_pipeline)
- Decode buffer handling
- Simplify _chunked_decode_attention to only validate and delegate
- Remove _decode_ring_buffer_pipeline and _decode_with_layer_pipeline from attention.py
- Add supports_decode check for policy validation
This completes the SparsePolicy v5 refactoring where both prefill and
decode paths now delegate all computation to the sparse policy.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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