Extract channel width from RR graph

[soheilshahrouz]

This PR computes channel widths directly from the RR graph. These computed values now replace t_chan_width usages outside of the RR graph generation.

[amin1377]

Thanks, Soheil!

[soheilshahrouz]

QoR differs from master because the computed channel width for the top and right channels at the periphery of the device is zero, while on master, t_chan_width indicates their width as W. This affects the average number of tracks within a net’s bounding box, which is used by the NetCostHandler class. As a result, placement differs slightly from master, since nets with endpoints on the top or right edges of the device receive a marginally higher cost.

As discussed with @amin1377, we believe the current implementation is more accurate, since those channels actually contain no tracks. I’ll collect QoR results for this branch and update the golden results if no significant degradation is observed.

Currently, bounding box computation uses the tile location of pins. It would be more accurate to use the location of the channel the pin connects to. This would require subtracting one from the x/y coordinate of pins located on the left or bottom sides of a tile.

[vaughnbetz]

Should do a QoR run on this to ensure all is well. Suggest:
VTR
Titanium/Stratix 10 (has different channel widths)
Some 3D arch.