docstr: Performs a single optimization step.
code: def step(self, closure=None):
      """
      Performs a single optimization step.
      """
      self._sync_fp16_grads_to_fp32()
      self.optimizer.step(closure)
  
      # copy FP32 params back into FP16 model
      for p, p32 in zip(self.fp16_params, self.fp32_params):
          if not p.requires_grad:
              continue
          p.data.copy_(p32.data)
  
imports: import math
  from itertools import chain
  from typing import Optional
  import parlai.utils.logging
label_desc: Use image node_id 11 for calling the SafeFP16Optimizer obj's underlying member method code with example usage: obj.step(closure) without return types