Reimplement classification task

[remtav]

In current version of GDL, the only task available is segmentation

PR #208 removed train_classification and classification at inference remains implemented in inference_segmentation.py, but is unreachable due to the current usage of GDL.py using mode and task to execute a specific script.

[mpelchat04]

The issue name suggest we should reimplement it but since it has been deprecated, shouldn't we just make sure it has been removed properly (i.e. in every files)?

[CharlesAuthier]

yeah I think we can only remove it