CUDA graph: Fix gsa broadcast — contiguous for prefill, reshape for decode

The stride-0 expand view for gsa_gpu caused illegal memory access
in quantize_nvfp4_from_buffer kernel. The CUDA kernel may not handle
stride-0 tensors correctly.

Fix:
- M=1 decode (graph-captured): just reshape scalar to (1,) — no alloc
- M>1 prefill (not graph-captured): expand + contiguous — allocation OK

dsv4/ops/quantize.py

@@ -323,18 +323,16 @@ def quantize_nvfp4_gpu_fused(x_bf16, divisor=6.0 * 448.0):
     from dsv4.kernels.cuda.loader import get_cuda_module
     amax_mod = get_cuda_module("amax_gsa", ["amax_gsa.cu"])
     gsa_gpu = amax_mod.compute_amax_gsa(x_bf16, divisor)  # scalar GPU tensor
-    # Broadcast to (M,) for the quantize-from-buffer kernel
-    # CUDA-graph-safe: use reshape+expand without .contiguous() allocation.
-    # For M=1 decode (the common graph-captured case), gsa is already scalar — no alloc.
-    # For M>1 prefill (not graph-captured), expand creates a view, and the CUDA kernel
-    # reads it correctly because the underlying data is contiguous (single value expanded).
-    # If the kernel truly requires physical contiguity, the caller should pre-allocate
-    # a buffer and use copy_ instead.
+    # Broadcast to (M,) for the quantize-from-buffer kernel.
+    # CUDA-graph-safe approach:
+    # - For M=1 decode (graph-captured): just reshape to (1,) — no allocation.
+    # - For M>1 prefill (not graph-captured): expand + contiguous is fine.
     M = x_bf16.shape[0]
     if gsa_gpu.dim() == 0:
-        gsa_gpu = gsa_gpu.reshape(1).expand(M)  # (M,) view — no allocation
-    elif gsa_gpu.shape[0] == 1 and M > 1:
-        gsa_gpu = gsa_gpu.expand(M)  # view — no allocation
+        gsa_gpu = gsa_gpu.reshape(1)  # scalar → (1,) — no allocation
+    if M > 1:
+        gsa_gpu = gsa_gpu.expand(M).contiguous()  # (M,) — allocation OK for prefill
+    # For M=1: gsa_gpu is (1,) contiguous — zero allocation
     quant_mod = get_cuda_module("fused_amax_quantize", ["fused_amax_quantize.cu"])
     x_fp4, x_sf = quant_mod.quantize_nvfp4_from_buffer(x_bf16, gsa_gpu)
     return x_fp4, x_sf, gsa_gpu