Assignment preprocesses: Fix domain content keys

[enrubio]

trafficstars

• Fixes the issue mentioned here: https://github.com/openreview/openreview-py/pull/2201#discussion_r1700795788

In some preprocess functions we retrieve fields from the domain using the committee name in the key. The problem is that these fields are hardcoded and don't use the committee name. It's an issue for venues that use custom role names.