multi_dataset breaks interpolation

[ibro45]

E.g.

    
val:
    freq: 1000

    multi_dataset:
        lungs:
            name: "CBCTtoCTValTestDataset"
            root: "/workspace/train_val/val"
        
        phantoms:
            name: "CBCTtoCTValTestDataset"
            root: "/workspace/train_val/val_phantom"

test:
    dataset: "${val.multi_dataset}"
    sliding_window: "${val.sliding_window}"

build_loader selects a single dataset from multi_dataset, assigns it to dataset and sets multi_dataset to None. This breaks interpolation when it's referring to multi_dataset.

Error:

omegaconf.errors.ConfigKeyError: str interpolation key 'val.multi_dataset' not found               
        full_key: test.dataset                                                                                                   
        reference_type=Optional[Dict[Union[str, Enum], Any]]                                                                      
        object_type=dict