GitLab

* Add bottom2gravity and balance_class option.

* Add valid_flag.

* Add use_valid_flag.

* Change balance_class option.

* Add wrapper, remove gravity and balance_class option.

* Change flag, change load_infos to repeat_indices.

* Remove pdb, change docstring.

* Change name, remove unnecessary args.

* Remove _concat_dataset.

* Change name of ClassSampledDataset.

* Change docstring.

* Add get_cat_ids in nus_dataset, add unittest for wrapper.

* Change to len(self.dataset)

* Update nus data.