Fix for custom configs not resolving correctly

[rdyro]

Creating custom configs in graphgym/contrib does not currently work correctly as far as I can tell.

Trying to specify example_arg or example_group -- arguments from the example contrib config -- yields an error as the config does not have those keys resolved yet, so cfg merge method refuses to merge unrecognized keys.

Traceback (most recent call last):
  File "/home/rdyro/Desktop/GraphGym/run/main.py", line 23, in <module>
    load_cfg(cfg, args)
  File "/home/rdyro/.pyenv/versions/devel/lib/python3.9/site-packages/graphgym/config.py", line 517, in load_cfg
    cfg.merge_from_file(args.cfg_file)
  File "/home/rdyro/.pyenv/versions/devel/lib/python3.9/site-packages/yacs/config.py", line 213, in merge_from_file
    self.merge_from_other_cfg(cfg)
  File "/home/rdyro/.pyenv/versions/devel/lib/python3.9/site-packages/yacs/config.py", line 217, in merge_from_other_cfg
    _merge_a_into_b(cfg_other, self, self, [])
  File "/home/rdyro/.pyenv/versions/devel/lib/python3.9/site-packages/yacs/config.py", line 491, in _merge_a_into_b
    raise KeyError("Non-existent config key: {}".format(full_key))
KeyError: 'Non-existent config key: example_arg'

The issue seems to have to do these line within the set_cfg function

for func in register.config_dict.values():
    func(cfg)

which operate over an empty dictionary, because graphgym.contrib has not yet been imported at that stage. However, importing graphgym.contrib directly in the config file leads to another set of errors as a couple of examples in graphgym.contrib rely on cfg having been initialized to the default structure.

The fix appears to require:

1. setting the cfg tree to a default structure with set_cfg(cfg)
2. importing the gaphgym.contrib tree
3. rerunning the following lines in the global file definition to now properly register custom config fields

   for func in register.config_dict.values():
   func(cfg)

[JiaxuanYou]

Thanks for the fix! It makes sense.