Data: add dim tag format stag-single:<idx>:<name>

[vieting]

I added the format stag-single:<idx>:<name> for dim tags to be able to distinguish spatial axes in pytorch-to-returnn. idx is relative to the matching stags, i.e., it is the index among the list of spatial dims matching name.

[vieting]

fixed issue with python 2 compatibility

[vieting]

Find related PR in pytorch-to-return here rwth-i6/pytorch-to-returnn#36

[albertz]

Btw, stag-single is the first prefix I came up with. But I'm not sure if this is really good. Do you maybe have better ideas? Or you think it's ok?

[vieting]

I think it's okay. Maybe something like stag-unique or stag-index otherwise? I added the comments to make clear what this is intended for.