Added a flag to gnn_explainer.

[wsad1]

#2203.

Made changes to.

• gnn_explainer.py
• • Added a flag called retrun_type to gnn_explainer. This flag indicates if the model will output raw scores, probabilities or log of probability.
• • Within the explian_node module model outputs are converted to log of probability based on the flag. to_log_proba is used to convert outputs.
• test_gnn_explainer.py: Add a new test function to test to_log_proba

[rusty1s]

Awesome! Thanks a lot :)