message: Update partition_assignment error handling
  Now handles -1 value for an unset parameter
diff: @@ -43,7 +43,7 @@ int64 ThreadsPerBlockLimit(const se::DeviceDescription& device_desc) {
  auto threads_per_core = device_desc.threads_per_core_limit();
  auto blocks_per_core = device_desc.blocks_per_core_limit();
  int64 threads_per_block;
  - if (threads_per_core != 0 && blocks_per_core != 0) {
  + if (threads_per_core > 0 && blocks_per_core > 0) {
  threads_per_block = device_desc.threads_per_core_limit() /
  device_desc.blocks_per_core_limit();
  } else {
  