def collate_fn(data):
"""Creates mini-batch tensors from the list of tuples (src_seq, trg_seq).
We should build a custom collate_fn rather than using default collate_fn,
because merging sequences (including padding) is not supported in default.
Seqeuences are padded to the maximum length of mini-batch sequences (dynamic padding).
Args:
data: list of tuple (src_seq, trg_seq).
- src_seq: torch tensor of shape (?); variable length.
- trg_seq: torch tensor of shape (?); variable length.
Returns:
src_seqs: torch tensor of shape (batch_size, padded_length).
src_lengths: list of length (batch_size); valid length for each padded source sequence.
trg_seqs: torch tensor of shape (batch_size, padded_length).
trg_lengths: list of length (batch_size); valid length for each padded target sequence.
"""
def merge(sequences):
lengths = [len(seq) for seq in sequences]
padded_seqs = torch.zeros(len(sequences), max(lengths)).long()
for i, seq in enumerate(sequences):
end = lengths[i]
padded_seqs[i, :end] = seq[:end]
return padded_seqs, lengths
# sort a list by sequence length (descending order) to use pack_padded_sequence
data.sort(key=lambda x: len(x[0]), reverse=True)
# seperate source and target sequences
src_seqs, trg_seqs = zip(*data)
# merge sequences (from tuple of 1D tensor to 2D tensor)
src_seqs, src_lengths = merge(src_seqs)
trg_seqs, trg_lengths = merge(trg_seqs)
return src_seqs, src_lengths, trg_seqs, trg_lengths
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