Bundles should handle zero validation input

Is your feature request related to a problem? Please describe.
For both single gpu and multi-gpu training, if the validation input is empty or not enough (in case of multi-gpu distribution) currently it throws an error. However it should allow and skip the Evaluation step if there is no enough data for validation. (example spleen bundle currently fails)

Describe the solution you'd like
Many times, user has little data to finetune and existing model. And he/she doesn't have validate split or wants to skip.

Additional context
Such situations normally happen during Active Learning iterations. Where annotators add small set of labels per iteration in the beginnning.