[tensor-shapes] Add `abs` to the DSL, at least for Flag[int]

[stroxler]

Describe the Bug

We currently have some ugly if/else conditions, for example in diag / diagonal implementations, because of lack of abs.

Adding abs for Flag[int] should be simple and a good UX improvement. A more interesting question is whether it would be worth adding symbolic abs but I'm not planning to do that right now.