Modify `TrainProperties` in `monai.bundle.properties`

"train_dataset_data.BundleProperty.REQUIRED" should be `False` to support datasets in `monai.apps.datasets`.

related discussion:
https://github.com/Project-MONAI/model-zoo/pull/452#issuecomment-1584810096