0a697f5871
Direct translation of CCCL dispatch_scan.cuh (1469 lines) architecture:
CCCL dispatch_scan has two kernels:
1. DeviceScanInitKernel — initializes tile_state (parallelizable)
2. DeviceScanKernel — sequential scan using tile_state propagation
Our _torch_chunk_gated_delta_rule now separates:
Phase 1 (init, parallelizable): pre-compute ALL chunk-local attn matrices
attn_i[c] = q[c] @ k[c].T * decay[c] — does NOT depend on state
Also pre-compute g.exp() and clamped g once, outside loop
Phase 2 (scan, sequential): only state-dependent ops in the loop
v_prime, v_new, attn_inter, core_out, state update
This matches CCCL's insight: everything that doesn't need tile_state
should be computed before the scan kernel, not interleaved with it.