fix cublas on h100

mlx/backend/cuda/gemms/cublas_gemm.cpp

@@ -213,7 +213,7 @@ void Matmul::run_impl(
         matmul_desc_,
         a_desc_,
         b_desc_,
-        out_desc_, // TODO should that be c_desc is it's set?
+        c ? c_desc_ : out_desc_,
         out_desc_,
         pref_,
         1,
@@ -226,8 +226,10 @@ void Matmul::run_impl(
 
   void* workspace_ptr = nullptr;
   if (heuristic_.workspaceSize > 0) {
+    // Ensure workspace is 256-byte aligned
+    int nbytes = cuda::ceil_div(heuristic_.workspaceSize, 256) * 256;
     array workspace(
-        allocator::malloc(heuristic_.workspaceSize),
+        allocator::malloc(nbytes),
         {static_cast<int>(heuristic_.workspaceSize)},
         int8);
     encoder.add_temporary(workspace);