Rename bundled dataset shards to match read_data

[iorinu]

Hello, I tried to reproduce the GPFL experiment, but the file names didn't match, so I made some corrections. I believe the functions were likely modified to align with PFLlib, so I've updated the filenames in the dataset.

def read_data(dataset, idx, is_train=True):
    if is_train:
        train_data_dir = os.path.join('../dataset', dataset, 'train/')

        train_file = train_data_dir + str(idx) + '.npz'
        with open(train_file, 'rb') as f:
            train_data = np.load(f, allow_pickle=True)['data'].tolist()

        return train_data

train_file = train_data_dir + str(idx) + '.npz'
I'm making adjustments so that it fits this code.