Fix batch norm defaults

[albertz]

They are wrong in RETURNN. But we can fix them here.

[albertz]

We can also fix them on RETURNN side: rwth-i6/returnn#522
Edit This is done now on RETURNN side.

And then maybe via #84. Edit Also done.

[albertz]

When we just reimplement batch norm here in returnn-common (#89), of course we also should fix them as well. The question is, what defaults to use then. Maybe the new defaults as discussed in rwth-i6/returnn#522.

[albertz]

The question is also about masked_time (or whatever equivalent option we would have here). masked_time=False could have a big impact on the runtime performance efficiency, and is even the default in all other frameworks, even though it is a bit incorrect.

I would say that masked_time maybe should even be mandatory when there are dynamic dims, to be explicit about this behavior.

See also here for some discussion on RETURNN side.

[albertz]

We wrapped the RETURNN BatchNormLayer now (see #89), and that one has the new defaults now (rwth-i6/returnn#522).