Enforce dim tags to be unique?

[albertz]

Related is #17 on how we handle dim tags in general here.

This needs rwth-i6/returnn#632 on the RETURNN side. Which is maybe a flag like behavior_unique_dim_tags = True.

We could just always enable this flag here. See the discussion in rwth-i6/returnn#632 why this might be useful.

I think this also would imply that we always need to have dim tags explicitly (so never just an integer for n_out) but I'm not exactly sure. This is again #17.

[albertz]

We mostly have this already now on returnn-common. E.g. nn.Parameter enforces it. dot only has the reduce argument, so it is not possible that you end up with non-unique dim tags. There might be some other rare cases. But then, as we have dim tags mandatory, it would fail very soon as when you match a non-unique dim tag, it would also error.

So I think this can be closed for now.