Update fit bot for 4.0.8

Created by: scarlehoff

And use replica-by-replica fit.

Up to #1881 this was completely unchanged, see https://github.com/NNPDF/nnpdf/pull/1920#issuecomment-1913302769)

However, after #1881 it changed due to a bug in which weights were saved as reference. Upon fixing this bug the fitbot hopefully remains unchanged.