Fix utils and add SAMPLE_RATIO

[iamyifan]

• Add SAMPLE_RATIO in config.py as a predefined parameter to allow users to sample from a large dataset.
• Decorate utility functions in utils.py with @staticmethod in BabyJoeyUtils.
• Modify main.py to integrate with the new parameter SAMPLE_RATIO.
• Rewrite sample_dataset in BabyJoeyUtils by using dataset.select method (Hugging Face Doc).