nn.dropout should have axis mandatory

[albertz]

Related: #69, #76

[albertz]

I'm not so sure anymore. Using the feature dim is very standard for dropout.

You might argue that we never should use this concept of default feature dim. However, it is used in many places, even e.g. nn.Linear (#75). Also, as it was discussed before (rwth-i6/returnn#586 (comment)), maybe having a feature dim is actually not too much a problem. Except for those rare cases where it can become ambiguous but then we should just require those to be non-ambiguous.

But this discussion does not really belong here but to #17 instead.

I just say, if we make use in some places, e.g. also nn.Linear, then it should be ok here as well.

(@Zettelkasten opinions maybe?)

[albertz]

Well, actually I think we should just make it mandatory anyway. Done now.