Remove/change the default "target" layer option for losses

[albertz]

#508

Effectively this means changing Loss.get_default_target() (because each loss class already can define its own default). The base function currently has this implementation:

  @classmethod
  def get_default_target(cls, extern_data):
    """
    :param TFNetwork.ExternData extern_data:
    :return: default target name, or None if this loss does not have a target
    :rtype: str|None
    """
    if not cls.need_target:
      return None
    return extern_data.default_target

The extern_data.default_target can be configured by the user via default_target in the config, and by default it is "classes".

Maybe just completely remove this function. So for losses which need a target (have need_target set), we explicitly require the target. So then effectively there is no default "classes" anymore.

[albertz]

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