nn.pad should output new dim tag

[albertz]

All functions which create a new dim tag output that new dim tag (for example nn.merge_dims or whatever). nn.pad is inconsistent in that it does not.

[albertz]

@JackTemaki @Atticus1806 agree? This probably would break some existing setups.

[Atticus1806]

It would break a network for me, but I would not mind since it needs to be updated next time I update returnn_common anyways and this is nothing that would go unnoticed. I think the changes which potentially can go unnoticed are the dangerous ones, things like this are fine for now.

[albertz]

Yea ok. Although, even if a change is dangerous, I think currently the priority would be on what is effectively the better API in the end, so this is also my main question here, what actually makes more sense.

(But anyway, I think the number of breaking changes up to first stable release is not much anymore, and after that, we would not do any breaking changes anymore anyway.)