metal lowbit kernels: check contiguity of scales and zeros

torchao/experimental/kernels/mps/test/test_lowbit.mm

@@ -101,7 +101,8 @@ void init() {
     int32_t ceil_K_group_size = (K + qGroupSize - 1) / qGroupSize;
     for (int idx = 0; idx < N * ceil_K_group_size; ++idx) {
       s_ptr[idx] = (idx + 1.0) / N;
-      z_ptr[idx] = int_distrib(generator);
+      auto zp = int_distrib(generator);
+      z_ptr[idx] = -zp * s_ptr[idx];
     }
     for (int idx = 0; idx < M * N; ++idx) {
       c_ptr[idx] = -1.0;

torchao/experimental/ops/mps/register.mm

@@ -58,13 +58,15 @@ void check_linear_mps_args(
       ": expect S to be 2d tensor with shape [:, ",
       N,
       "]");
+  TORCH_CHECK(S.is_contiguous(), __func__, " : expect S to be contiguous.");
 
   TORCH_CHECK(
       Z.dim() == 2 && Z.size(1) == N,
       __func__,
       ": expect Z to be 2d tensor with shape [:, ",
       N,
       "]");
+  TORCH_CHECK(Z.is_contiguous(), __func__, " : expect Z to be contiguous.");
 }
 
 template <int nbit>

torchao/experimental/ops/mps/test/test_lowbit.py

@@ -46,18 +46,18 @@ class TestLowBitQuantWeightsLinear(unittest.TestCase):
     ]
 
     def _init_tensors(self, group_size, M, K, N, nbit, device="mps"):
-        max_abs = 1 << (nbit - 1)
         ceil_K_group_size = (K + group_size - 1) // group_size
-        A = 2 * torch.rand(M, K, dtype=torch.float32, device=device) - 1
-        W = torch.randint(0, 2 * max_abs, (N, K), dtype=torch.uint8, device=device)
+        A = torch.rand(M, K, dtype=torch.float32, device=device)
+        W = torch.randint(0, 1 << nbit, (N, K), dtype=torch.uint8, device=device)
         S = torch.rand(ceil_K_group_size, N, dtype=torch.float32, device=device) + 0.01
         Z = torch.randint(
             0,
-            2 * max_abs,
+            1 << nbit,
             (ceil_K_group_size, N),
             dtype=torch.float32,
             device=device,
         )
+        Z = -Z * S
         return A, W, S, Z
 
     def _reference_linear_lowbit_quant_weights(self, A, W, group_size, S, Z, nbit):