BUG: default value of fusion should be set to None #17
Description
https://github.com/mahmoodlab/MCAT/blob/b9cca63be83c67de7f95308d54a58f80b78b0da1/main.py#L106C1-L107C1 #16
The default setting of fusion now is concat, which will cause "float division by zero" error for WSI only baselines.