Fixes # https://gitee.com/mindspore/dashboard?issue_id=I1T7US

Special notes for your reviewers:

Glossary of changes:

1. Python API in dataset.py PYBIND
2. CPP constructor/get_dataset_size
3. add test case to python
4. CPP/Python validator/builder
5. refactor arg names of CelebADataset and VOCDataset