message: relax 'extra_dep' argument in CUDA scatter/gather
diff: @@ -737,7 +737,6 @@ ENOKI_EXPORT uint32_t cuda_trace_append(EnokiType type,
  ctx.live.insert(idx);
  if (v.cmd.find("ld.global") != std::string::npos) {
  - assert(ctx.scatter_gather_operand != 0);
  v.extra_dep = ctx.scatter_gather_operand;
  cuda_inc_ref_ext(v.extra_dep);
  } else {
  @@ -794,7 +793,6 @@ ENOKI_EXPORT uint32_t cuda_trace_append(EnokiType type,
  ctx.live.insert(idx);
  if (v.cmd.find("st.global") != std::string::npos) {
  - assert(ctx.scatter_gather_operand != 0);
  v.extra_dep = ctx.scatter_gather_operand;
  cuda_inc_ref_ext(v.extra_dep);
  } else {
  