Bundle sparse_sage Triton kernel for block-sparse attention

Without sparse attention, the model uses full (dense) attention which
attends to distant irrelevant information, causing ghosting artifacts.
The FlashVSR paper explicitly requires block-sparse attention.

Vendored from SageAttention team (Apache 2.0), pure Triton (no CUDA C++).
Import chain: local sparse_sage → external sageattn.core → SDPA fallback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flashvsr_arch/models/sparse_sage/quant_per_block.py

@@ -0,0 +1,110 @@
+"""
+Per-block INT8 quantization kernel for Sparse SageAttention.
+
+Copyright (c) 2024 by SageAttention team.
+Licensed under the Apache License, Version 2.0
+"""
+
+import torch
+import triton
+import triton.language as tl
+
+
+@triton.jit
+def quant_per_block_int8_kernel(
+    Input, Output, Scale, L,
+    stride_iz, stride_ih, stride_in,
+    stride_oz, stride_oh, stride_on,
+    stride_sz, stride_sh,
+    sm_scale,
+    C: tl.constexpr, BLK: tl.constexpr,
+):
+    off_blk = tl.program_id(0)
+    off_h = tl.program_id(1)
+    off_b = tl.program_id(2)
+
+    offs_n = off_blk * BLK + tl.arange(0, BLK)
+    offs_k = tl.arange(0, C)
+
+    input_ptrs = (
+        Input
+        + off_b * stride_iz
+        + off_h * stride_ih
+        + offs_n[:, None] * stride_in
+        + offs_k[None, :]
+    )
+    output_ptrs = (
+        Output
+        + off_b * stride_oz
+        + off_h * stride_oh
+        + offs_n[:, None] * stride_on
+        + offs_k[None, :]
+    )
+    scale_ptrs = Scale + off_b * stride_sz + off_h * stride_sh + off_blk
+
+    x = tl.load(input_ptrs, mask=offs_n[:, None] < L)
+    x = x.to(tl.float32)
+    x *= sm_scale
+    scale = tl.max(tl.abs(x)) / 127.0
+    x_int8 = x / scale
+    x_int8 += 0.5 * tl.where(x_int8 >= 0, 1, -1)
+    x_int8 = x_int8.to(tl.int8)
+    tl.store(output_ptrs, x_int8, mask=offs_n[:, None] < L)
+    tl.store(scale_ptrs, scale)
+
+
+def per_block_int8(q, k, km=None, BLKQ=128, BLKK=64, sm_scale=None, tensor_layout="HND"):
+    q_int8 = torch.empty(q.shape, dtype=torch.int8, device=q.device)
+    k_int8 = torch.empty(k.shape, dtype=torch.int8, device=k.device)
+
+    if km is not None:
+        k = k - km
+
+    if tensor_layout == "HND":
+        b, h_qo, qo_len, head_dim = q.shape
+        _, h_kv, kv_len, _ = k.shape
+        stride_bz_q, stride_h_q, stride_seq_q = q.stride(0), q.stride(1), q.stride(2)
+        stride_bz_qo, stride_h_qo, stride_seq_qo = q_int8.stride(0), q_int8.stride(1), q_int8.stride(2)
+        stride_bz_k, stride_h_k, stride_seq_k = k.stride(0), k.stride(1), k.stride(2)
+        stride_bz_ko, stride_h_ko, stride_seq_ko = k_int8.stride(0), k_int8.stride(1), k_int8.stride(2)
+    elif tensor_layout == "NHD":
+        b, qo_len, h_qo, head_dim = q.shape
+        _, kv_len, h_kv, _ = k.shape
+        stride_bz_q, stride_h_q, stride_seq_q = q.stride(0), q.stride(2), q.stride(1)
+        stride_bz_qo, stride_h_qo, stride_seq_qo = q_int8.stride(0), q_int8.stride(2), q_int8.stride(1)
+        stride_bz_k, stride_h_k, stride_seq_k = k.stride(0), k.stride(2), k.stride(1)
+        stride_bz_ko, stride_h_ko, stride_seq_ko = k_int8.stride(0), k_int8.stride(2), k_int8.stride(1)
+    else:
+        raise ValueError(f"Unknown tensor layout: {tensor_layout}")
+
+    q_scale = torch.empty(
+        (b, h_qo, (qo_len + BLKQ - 1) // BLKQ), device=q.device, dtype=torch.float32,
+    )
+    k_scale = torch.empty(
+        (b, h_kv, (kv_len + BLKK - 1) // BLKK), device=q.device, dtype=torch.float32,
+    )
+
+    if sm_scale is None:
+        sm_scale = head_dim ** -0.5
+
+    grid = ((qo_len + BLKQ - 1) // BLKQ, h_qo, b)
+    quant_per_block_int8_kernel[grid](
+        q, q_int8, q_scale, qo_len,
+        stride_bz_q, stride_h_q, stride_seq_q,
+        stride_bz_qo, stride_h_qo, stride_seq_qo,
+        q_scale.stride(0), q_scale.stride(1),
+        sm_scale=(sm_scale * 1.44269504),
+        C=head_dim, BLK=BLKQ,
+    )
+
+    grid = ((kv_len + BLKK - 1) // BLKK, h_kv, b)
+    quant_per_block_int8_kernel[grid](
+        k, k_int8, k_scale, kv_len,
+        stride_bz_k, stride_h_k, stride_seq_k,
+        stride_bz_ko, stride_h_ko, stride_seq_ko,
+        k_scale.stride(0), k_scale.stride(1),
+        sm_scale=1.0,
+        C=head_dim, BLK=BLKK,
+    )
+
+    return q_int8, q_scale, k_int8, k_scale