Enhance the handler `ProbMapProducer` on checking/making output dir

Currently, if `output_dir` is not existing, an error will be raised. I think a better way to address it is to do sth like the `SaveImaged` transform does (see: https://github.com/Project-MONAI/MONAI/blob/a8f67e4278d46b49a87488a41ba38431883e8907/monai/data/folder_layout.py#L95)