    def build_optimizer(cls, cfg: DictConfig, params, **kwargs):
        """
        Args:
            cfg (omegaconf.DictConfig): fairseq args
            params (iterable): iterable of parameters to optimize
        """
        flatten = not getattr(cfg.common, "fp16_no_flatten_grads", False)
        if getattr(cfg.common, "bf16", False):
            flatten = False  # mixed precision is faster on TPUs without flat grads
        fp32_params = cls.build_fp32_params(cfg.optimizer, params, flatten=flatten)
        if flatten:
            fp32_optimizer = optim.build_optimizer(cfg.optimizer, [fp32_params])
        else:
            fp32_optimizer = optim.build_optimizer(cfg.optimizer, fp32_params)
        if flatten and not fp32_optimizer.supports_flat_params:
            raise RuntimeError(
                f"chosen optimizer {fp32_optimizer.__class__.__name__} does not support flat params, please set --fp16-no-flatten-grads"
            )
        return cls(cfg, params, fp32_optimizer, fp32_params, **kwargs)
Args:
cfg (omegaconf.DictConfig): fairseq args
params (iterable): iterable of parameters to optimize
fairseq/optim/fp16_optimizer.py