Implement dBias gradient computation in CUDA backward kernel

[Copilot]

This PR implements the missing bias gradient (dBias) calculation in the CUDA backward kernel compute_dq_dk_dv_1colblock function, referencing the dBias calculation logic from the Triton implementation as requested in the issue.

Changes Made

Core Implementation

Added dBias computation following the exact Triton formula:

// Following Triton logic: dbias = p * (dp - Di[:, None])
dBias(mi, ni) = scores(mi, ni) * (dS(mi, ni) - dP_sum(mi));

Where:

• scores = attention probabilities (p)
• dS = gradient from dO @ V^T computation (dp)
• dP_sum = diagonal sums (Di)

Memory Management

Implemented proper data flow using existing infrastructure:

• Registers → Shared Memory: Uses existing smem_thr_copy_PdS operations
• Shared Memory → Global Memory: Uses GmemTiledCopyBias with proper bounds checking
• Synchronization: Added __syncthreads() before global memory copy
• Pointer Management: Advances gdBias pointer by kBlockM * params.dbias_row_stride per iteration

Key Design Decisions

1. Timing: dBias computed BEFORE pointwise_mult modifies dS (critical for correctness)
2. Infrastructure Reuse: Leverages existing copy operations since bias has same shape as P/dS
3. Pattern Compliance: Follows established patterns from dK/dV copy operations
4. Bounds Checking: Uses the same bounds checking logic as other gradient tensors

Validation

Mathematical correctness verified with Python test scripts:

• ✅ dBias formula matches Triton implementation exactly
• ✅ Block-wise computation equivalent to nested loop pattern
• ✅ Edge cases (zero scores, identity matrices) handled correctly
• ✅ All required infrastructure (GmemTiledCopyBias, SmemLayoutBias, etc.) confirmed to exist

Integration

The implementation integrates seamlessly with existing code:

• Uses pre-existing gdBias tensor and dbias_* parameters from Flash_bwd_params
• Leverages existing shared memory layouts and copy atoms
• Follows the same memory access patterns as other gradient computations

Files Modified

• csrc/src/flash_bwd_kernel.h: Added ~44 lines implementing dBias calculation and memory operations

Testing Status

• ✅ Mathematical logic validation complete
• ✅ Code structure and infrastructure compatibility verified
• ⚠️ CUDA compilation testing pending (requires CUDA environment)
• ⚠️ End-to-end numerical validation pending (requires full build)

The implementation is ready for CUDA compilation and should provide equivalent results to the Triton backend.

Fixes #72.

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