DecideLayer fix out dim tag

[albertz]

No description provided.

[albertz]

  File "/home/runner/work/returnn/returnn/returnn/tf/layers/rec.py", line 5230, in DecideLayer.decide
    line: tag = tag.get_for_batch(output.batch)
    locals:
      tag = <local> DimensionTag{'time:var:extern_data:classes'[B&Beam{'output/output'}(4)]}
      tag.get_for_batch = <local> !AttributeError: 'DimensionTag' object has no attribute 'get_for_batch'
      output = <local> Data(name='decision_output', dtype='int32', sparse=True, dim=10, batch_shape_meta=[B,T|'time:var:extern_data:classes'[B&Beam{'output/output'}(4)]])
      output.batch = <local> BatchInfo{B}
AttributeError: 'DimensionTag' object has no attribute 'get_for_batch'

So this needs #613 first.

[albertz]

This was merged as part of #613 now.