[ci] Add clarity to `amdgpu_family_matrix.py` for presubmit, postsubmit and nightlies

[geomin12]

Currently, amdgpu_family_matrix.py is unclear on how presubmit, postsubmit and nightlies are selected. If there are any duplicates, this causes issues in the code. For nightlies too with #2200 , all archs are selected but the python does not reflect that correctly

Eventually, we want a function such as:

def selector(amdgpu_target, submit_target):
     ...

where the user would just need to add their gpu target and submit target (ex: presubmit, nightlies, etc)

[HereThereBeDragons]

I eventually am working on a refactoring of this to enable CI weekly. #1732
It has the idea to have only a single dictionary with all the machines and that you have a list for each of the different run types (pre/post/nightly)