fix sequence_mask with None lengths

[hscspring]

the code location: encoders/transformer.py:

class TransformerEncoder(EncoderBase):
    ...
    def forward(self, src, lengths=None):
        """See :func:`EncoderBase.forward()`"""
        self._check_args(src, lengths)

        emb = self.embeddings(src)

        out = emb.transpose(0, 1).contiguous()
        mask = ~sequence_mask(lengths).unsqueeze(1)
        # Run the forward pass of every layer of the tranformer.
        for layer in self.transformer:
            out = layer(out, mask)
        out = self.layer_norm(out)

here when lengths is None, mask = ~sequence_mask(lengths).unsqueeze(1) will return a AttributeError: 'NoneType' object has no attribute 'numel', so it means that I should always need to pass a lengths here to replace the default None.

[francoishernandez]

Thanks for this but what's the use case // did it cause an issue // in what kind of setup?

[hscspring]

very sorry for that, i should make a clear explain.