docstr: Save model to checkpoint but only on master process.
code: def save_unsharded_model(
      self,
      model: ModelWrapper,
      checkpoint: str,
      gather_dtensor: bool,
      use_safetensors: bool,
  ):
      """
      Save model to checkpoint but only on master process.
      """
      assert isinstance(
          model, ModelWrapper
      ), "Please boost the model before saving!"
      if self.coordinator.is_master():
          super().save_unsharded_model(
              model.unwrap(),
              checkpoint,
              gather_dtensor,
              use_safetensors,
          )
  
imports: from typing import Callable, Iterator, List, Optional, Tuple
  import torch.nn
  from torch.nn.parallel import DistributedDataParallel
  from torch.optim import Optimizer
  from torch.optim.lr_scheduler import _LRScheduler
  from torch.utils.data import DataLoader
  from colossalai.checkpoint_io import CheckpointIO, GeneralCheckpointIO
  from colossalai.cluster import DistCoordinator
  from colossalai.interface import ModelWrapper, OptimizerWrapper
  from .dp_plugin_base import DPPluginBase
label_desc: Use image node_id 3 for calling the TorchDDPCheckpointIO obj's underlying member method code with example usage: obj.save_unsharded_model(model, checkpoint, gather_dtensor, use_safetensors) without return types