    def zero_grad(self):
        """Clears the gradients of all optimized parameters."""
        for p in self.fp16_params:
            p.grad = None
        if self.has_flat_params:
            if torch.is_tensor(self.fp32_params):
                self.fp32_params.grad.zero_()
            elif isinstance(self.fp32_params, dict):
                for fp32_params in self.fp32_params.values():
                    fp32_params.grad.zero_()
            else:
                raise RuntimeError("self.fp32_params must be a tensor or dict")
        else:
            for p32 in self.fp32_params:
                if p32.grad is not None:
                    p32.grad.zero_()
        self._needs_sync = False

        if self.scaler is not None:
            self._multiply_factor = 1.0 / float(self.scaler.loss_scale)
Clears the gradients of all optimized parameters.
fairseq/optim/fp16_optimizer.py