Graphormer and dataset extensions

Four extensions, many thanks to @luis-mueller for spearheading this work!

Graphormer port with model config to reproduce the expected performance on ZINC
GPS can be run in a configuration with biased global attention module, i.e., with Graphormer-style model
Added heterophilic transductive dataset support: Actor, Cornell, Texas, Wisconsin, Squirrel, Chameleon
Added synthetic datasets

rampasek / GraphGPS