Fixes: #2927

This update calculates the number of iterations per epoch dynamically instead of hardcoding it to 250.

The method get_tr_and_val_datasets in the nnUNetTrainer class was modified to update both self.num_iterations_per_epoch and self.num_val_iterations_per_epoch based on the dataset size and batch size.

Additionally, progress bars were added to the training and validation loops to provide users with a clearer indication of progress during long training sessions.