`fit_with_thcovmat.yaml` in validphys examples doesn't seem valid

Created by: wilsonmr

I noticed this runcard in https://github.com/NNPDF/nnpdf/pull/1211/files?file-filters%5B%5D=.yaml#r632562581

Perhaps I missed something but I don't think nnfit ever got updated to use dataset_inputs and so I think this runcard isn't valid.

Am I wrong or does this runcard need to be changed to use BIGEXP