def sample_padding_fn(self, sample, size):
            """ pad sequence `seq` of shape SxE to size S'xE where S' is given by `size` """
            seq, label = sample
            seq = F.pad(seq, pad=(0, 0, 0, size - len(seq)), value=self.padding_value)
            return seq, label
 pad sequence `seq` of shape SxE to size S'xE where S' is given by `size` 
training/data_efficiency/variable_batch_size_and_lr/variable_batch_size_and_lr_example.py