TypeError: cannot pickle 'dict_keys' object  when distributed training

there is a subtle bug in 
https://github.com/nutonomy/nuscenes-devkit/blame/master/python-sdk/nuscenes/eval/detection/data_classes.py:39  
self.class_names = self.class_range.keys() 
when this line is pickled for pytorch distributed training , it will cause `TypeError: cannot pickle 'dict_keys' object`.
so it is necessary to change it to `self.class_names = list(self.class_range.keys())` .