Make sure a recent RETURNN behavior version is used?

[albertz]

In some cases, it might be required when we depend on this (not sure if we already have that).

In some cases, it would make sense. E.g. #83 could also be fixed this way.

The question is also, which behavior version to use, and how? Or maybe just requiring some minimum version, by checking for it in the network or so?

Not sure. Maybe also not needed.

[albertz]

The extended variant to define the RETURNN net dict and extern_data (#44, #56) will be generic to construct config options. This currently also includes behavior_version is just part of that. See NameCtx.get_returnn_config().