Fix vector dimension of OOV token (#275)

[domaala]

This fixes #275 .

[domaala]

The single test that was failing depended on Xavier initialization, which does not allow for 1-D tensors. Considering that we went for a 1-D vector of an OOV vector, I had to change it.

However, this is kind of problematic for unk_init in general, so we should at least update the documentation to explain this 1-D constraint.