Add dataset validation step and refactoring data modules

[illian01]

Description

Please include a summary of this pull request in English. If it closes an issue, please mention it here.

Closes: #253

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Change(s)

• Massive refactoring
  • Rename BaseSampler to BaseSampleLoader.
  • Define overlapping logics of various tasks in BaseSampleLoader.
  • Compress task specific modules to one file per task.
• Check dataset path at the start of build_dataset.
  • For train, train split of dataset must be provided, test split will be ignored.
  • For test, test split of dataset must be provided, train and valid split will be ignored.
• Check loaded dataset at the end of build_dataset.
  • log dataset volume.
  • If loaded dataset has no samples, raise error.

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