fixing issue with opacus

A bug has been found when calling Opacus dataloader batch_size attribute: it returns None, instead of the value of the current batch_size (see https://gitlab.inria.fr/fedbiomed/fedbiomed/-/blob/feature/fixing-fedprox-loss-display/fedbiomed/common/training_plans/_torchnn.py#L510 ) Actually, this opacus dataloader DPDataLoader returns batch of several size, but in average should equal the size of the batch specified in the training_data_loader

This branch should be merged after feature/fixing-fedprox-loss-display MR