fix checkpoints file list to align with DeepSpeed

[dc3671]

When use glob.glob(f"{self.model_path}/*.bin"), files path in the list will all contain model_path prefix. While set it as root_dir will not. And it will align to DeepSpeed's loading way (replace_module.py):

sd = [
                    torch.load(os.path.join(base_dir1,
                                            checkpoint[i]),
                               map_location='cpu')
        ]

Where base_dir1 is duplicate with model_path.

plz help review @mayank31398, thx~