RecLayer subnet output check format

Author: albertz

returnn/tf/layers/rec.py

@@ -2587,8 +2587,9 @@ def cond(i, net_vars, acc_tas, seq_len_info=None):
       if output_layer:
         assert isinstance(output_layer, LayerBase)
         output_data = output_layer.output.copy_as_time_major()
-        assert 0 in output_data.size_placeholder
-        rec_layer.output.size_placeholder = output_data.size_placeholder.copy()
+        # No need to copy size_placeholder, as we have dim tags now.
+        # However, we should check that the format is right.
+        assert rec_layer.output.dim_tags == output_data.dim_tags
         output = output_data.placeholder
       else:
         assert seq_len is not None