`model_outputs` as the counterpart to `extern_data`

[albertz]

It would define a number of outputs by your model. Just in the same format as extern_data.

This could be used by tools like the compile-TF-graph script. It can check that those output layers exist, and it can also check that the dimensions and type matches, and maybe automatically reorder the dims just as requested in model_outputs.

This came up for PyTorch ONNX export (https://github.com/rwth-i6/returnn/issues/1120#issuecomment-1286887444).