message: Initialize external_command_encoder_ to nil
  The uninitialized value can cause other functions (e.g., Invoke()) to
  behave as if the user has set an external command encoder.  Since the
  value is an invalid pointer, this causes the process to crash.
diff: @@ -184,6 +184,7 @@ class Delegate {
  options_.wait_type = TFLGpuDelegateWaitType::TFLGpuDelegateWaitTypePassive;
  }
  metal_device_ = MTLCreateSystemDefaultDevice();
  + external_command_encoder_ = nil;
  command_queue_ = [metal_device_ newCommandQueue];
  if (options_.wait_type == TFLGpuDelegateWaitType::TFLGpuDelegateWaitTypeAggressive) {
  gpu_alarm_clock_ = std::unique_ptr<GpuAlarmClock>(new GpuAlarmClock(command_queue_));
  