GEM: add_data to buffer

[tunglamlqddb]

https://github.com/aimagelab/mammoth/blob/f3effb9dc2098c0463d214c4f8e5e69f38517e13/models/gem.py#L126

Dear authors, This part where you add data to buffer in GEM is probably wrong because size of task_labels should be equal to that of examples, but not 'samples_per_task'. Thank you for revising this.