gpl: support regions for routability mode.

[gudeh]

Description

In the GPL code, NesterovBase consists of a class representing each region. We have the nbVec_ vector, which holds an object for each region. The first object in this vector (nbVec_[0]) contains all instances that do not belong to any specific region. For example, if a Verilog design is described without any defined regions, as is usually the case, GPL will hold all instances in nbVec_[0].

Currently, in routeBase.cpp, whenever we reference a NesterovBase object, we only do so for the object without regions (nbVec_[0]), while all other objects are ignored. For example:

OpenROAD/src/gpl/src/routeBase.cpp

Line 763 in 6c60401

nbVec_[0]->updateAreas();

I have not yet tested routability mode on a design with regions, but there is no way it would function correctly. However, I believe it will not necessarily throw an error.

Suggested Solution

• Test what currently happens on a design with regions (e.g., upf_aes in OpenROAD/test/).
• Most importantly, replace every instance of nbVec_[0] in routeBase.cpp with a loop iterating over nbVec_ objects, and move forward from there.

Additional Context

No response