Disallow data as layer input, always require data:<data_key_name>

[albertz]

#508

Thus effectively removing the default data key "data".

I don't have a strong opinion on this, so I just put this here and leave it open for discussion.

This is related but separate from #519 (default "from" layer arg).

[JackTemaki]

Strong agree, but I am also a person that started to use MetaDataset more often to simply be able to rename data as data key into someting more meaningful.

[albertz]

This would probably break most/all existing configs, so forcing users to change data to data:data (or however they data key is called). So this is one of such changes which requires slightly more effort for users to adopt. So some more people should probably be involved here to express their opinion. @rwth-i6/returnn

[albertz]

This here is a case where we do not necessarily need a new behavior_version and instead could just print a deprecation warning.