Add Blelloch parallel prefix scan for LASP

This PR implements Blelloch parallel prefix scan to reduce inter-GPU
communication from O(P) sequential steps (ring) to O(log P) parallel
steps (tree-based).

Key improvements:
- O(log P) communication complexity (e.g., 128 GPUs: 128 steps → 14 steps)
- Work-efficient tree-based algorithm
- Supports non-power-of-2 GPU counts
- Reuses KV/DKV buffers to avoid allocation overhead

Implementation details:

1. **BlellochScanner** (lasp/utils/blelloch_ops.py):
   - Tree-based up-sweep and down-sweep communication
   - Correct sender/receiver logic using "right edge" of subtrees
   - Distance-based decay in down-sweep for proper accumulation
   - Support for reverse scan (suffix) for backward pass
   - Global rank conversion for multi-group data parallelism

2. **lasp_blelloch** (lasp/lasp_blelloch.py):
   - Combines Blelloch scan with fused Triton kernels
   - Correct inclusive-to-exclusive conversion: λ^(-C) * (inclusive - local)
   - Buffer reuse pattern matching lasp_fuse_parallel
   - Forward: prefix scan, Backward: suffix scan

3. **Tests and benchmarks**:
   - test_blelloch_correctness.py: Gradient correctness tests
   - test_non_power_of_two.py: Non-power-of-2 world sizes
   - benchmark_blelloch.py: Performance benchmarks
   - benchmark_all_methods.py: Comprehensive comparison

Tested with:
- Single GPU and multi-GPU (4-8 GPUs)
- Data parallelism (dp_size > 1) with sequence parallelism
- Power-of-2 and non-power-of-2 world sizes
- Forward and backward pass correctness

Author: Hoang Phan

lasp/__init__.py

@@ -2,5 +2,6 @@
 from .lasp_fuse import *
 from .lasp_fuse_parallel import *
 from .lasp_naive import *
+from .lasp_blelloch import *
 from .lightning_attention import *
 from .utils import *