CP: make correct_attn_out robust to 4‑D views and fix Triton arg binding (#26509)

Signed-off-by: Huamin Li <3ericli@gmail.com>

Author: hl475

vllm/attention/ops/common.py

@@ -117,14 +117,52 @@ def correct_attn_out(
     if ctx is None:
         ctx = CPTritonContext()
 
-    lse = torch.empty_like(lses[0])
-
-    grid = (out.shape[0], out.shape[1], 1)
-    regular_args = (out, out, lses, lse, *out.stride(), *lses.stride(), cp_rank)
-    const_args = {
-        "HEAD_DIM": out.shape[-1],
-        "N_ROUNDED": lses.shape[0],
-    }
+    # --- Normalize to 3D views ---
+    if out.ndim == 4 and out.shape[1] == 1:
+        out = out.squeeze(1)
+    assert out.ndim == 3, f"expected out [B,H,D] or [B,1,H,D], got {tuple(out.shape)}"
+
+    if lses.ndim == 4 and lses.shape[-1] == 1:
+        lses = lses.squeeze(-1)
+    if lses.ndim == 4 and lses.shape[1] == 1:
+        lses = lses.squeeze(1)
+    assert lses.ndim == 3, (
+        f"expected lses [N,B,H] (optionally with a 1-sized extra dim), "
+        f"got {tuple(lses.shape)}"
+    )
+
+    B, H, D = out.shape
+    N = lses.shape[0]
+
+    # Strides after we normalized shapes to 3-D views.  The kernel computes
+    # offsets for `vlse_ptr` using lses_stride_B/H, so the output buffer must
+    # have the same B/H stride layout as a slice of `lses`.
+    o_sB, o_sH, o_sD = out.stride()
+    l_sN, l_sB, l_sH = lses.stride()
+
+    # Allocate LSE with the same B/H strides as `lses` so writes land correctly
+    # even when `lses` is a non-contiguous view (e.g., 4-D to 3-D squeeze).
+    lse = torch.empty_strided(
+        (B, H), (l_sB, l_sH), device=lses.device, dtype=lses.dtype
+    )
+
+    # Kernel launch config
+    grid = (B, H, 1)
+
+    regular_args = (
+        out,
+        out,
+        lses,
+        lse,
+        o_sB,
+        o_sH,
+        o_sD,
+        l_sN,
+        l_sB,
+        l_sH,
+        cp_rank,
+    )
+    const_args = {"HEAD_DIM": D, "N_ROUNDED": N}
 
     ctx.call_kernel(_correct_attn_cp_out_kernel, grid, *regular_args, **const_args)
     return out, lse