Fix erroneous relative path

[jonatan-hanssen]

openood/evaluation_api/evaluator.py uses filepath.split('/') on the absolute path of its own location, but this removes the first /, making the path relative instead of absolute. This means that openood/evaluation_api/postprocessor.py line 76 will always fail, because it is looking at $(pwd)/abolute/path/to/config as opposed to just the absolute path. It will then do a urllib.request and get the config file from github and create directories mirroring the absolute path in the current working directory. If you clone the repo in your home directory and create an Evaluator instance while in the repo, Evaluator will create the following directories and file: /home/user/OpenOOD/home/user/OpenOOD/configs/postprocessors/react.yml as opposed to getting react.yml from /home/user/OpenOOD/configs/postprocessors.