Features: Support multiple datasets

[seoulsky-field]

What

• Support for users to use multiple datasets.

Why

• It could exist that want to use multiple datasets such as both CheXpert and BRAX or all three datasets(CheXpert, MIMIC-CXR, BRAX). However, our current develop branch doesn't support to use multiple datasets.

How

• Support MIMIC-CXR dataset to work well.
• Support BRAX dataset to work well.
• Support multiple datasets (Use torch ConcatDataset method and revise hydra code to work well)