Finetuning : invalid name for group of parameters, to be changed (neural_net -> neural_net_hparams)

[CharlesGaydon]

https://github.com/IGNF/myria3d/blob/2260b080b6b152cd168d5cea86723a77f868dfcc/myria3d/train.py#L30C34-L30C34

This leads to mismatch in model dimensions when loading a pretrained model.

Additionnaly, the required method change_num_class_for_finetuning, that used to be part of the NN module, is not there anymore. This means that finetuning for a different number of classes is not supported (cf. finetuning callbacks)