[WIP] Builds on top of #969 — adds custom loss for classification tasks

[ltlinh16]

• Same idea as https://github.com/terrastackai/terratorch/pull/969, adding custom loss with custom loss kwargs option for classification tasks.
• Added unittest for classification tasks

[Joao-L-S-Almeida]

@ltlinh16 @romeokienzler , in this way, I think:

• It's better to close [WiP] Basic support for user-defined (and not included in TT) loss functions #969 and start with this new PR.
• To prepend [WiP] in the name of the PR while it's in construction and testing.
• The error in the tests comes from other issue. Maybe we should wait for PR Geobench must be installed #980 and Support for inference using Surya downstream models #967 be merged before considering to merge the current PR.