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

[yiheng-wang-nv]

"train_dataset_data.BundleProperty.REQUIRED" should be False to support datasets in monai.apps.datasets.

related discussion:
Project-MONAI/model-zoo#452 (comment)

[yiheng-wang-nv]

same for dataset_data in InferProperties

[yiheng-wang-nv]

An issue in:

MONAI/monai/bundle/workflows.py

Line 374 in e6ec945

if name in ("evaluator", "val_interval"):

local variable ref may not be assigned, but has to be referenced.