code: def update_main_grads(self):
      self._sync_fp16_grads_to_fp32()
  
imports: import math
  from itertools import chain
  from typing import Optional
  import parlai.utils.logging
label_desc: Use image node_id 9 for calling the SafeFP16Optimizer obj's underlying member method code with example usage: obj.update_main_grads() without return types