[WIP] Fix off-by-one error in numbering of epochs in most tutorials

The first epoch consisted in only one minibatch, and no validation
phase was used. For the following epochs, the validation phase happened
just before the last minibatch of the epoch.

I'm starting to re-run the examples in order to fix the documentation, as it changes the exact error, number of epochs actually used, and so on. I'm not sure if it is really worth it.