callate.py

def collate_fn(data):
	'''  
	We should build a custom collate_fn rather than using default collate_fn,
	as the size of every sentence is different and merging sequences (including padding) 
	is not supported in default. 
	Args:
		data: list of tuple (training sequence, label)
	Return:
		padded_seq - Padded Sequence, tensor of shape (batch_size, padded_length)
		length - Original length of each sequence(without padding), tensor of shape(batch_size)
		label - tensor of shape (batch_size)
    '''

    #sorting is important for usage pack padded sequence (used in model). It should be in decreasing order.
	data.sort(key=lambda x: len(x[0]), reverse=True)
	sequences, label = zip(*data)
	length = [len(seq) for seq in sequences]
	padded_seq = torch.zeros(len(sequences), max(length)).long()
	for i, seq in enumerate(sequences):
		end = length[i]
		padded_seq[i,:end] = seq
	return padded_seq, torch.from_numpy(np.array(length)), torch.from_numpy(np.array(label))