Support hetrogenous graphs

[ShaharGottlieb]

support heterogeneous graphs in explainer for all tasks.
requirements:

• support in GNNExaplainer
• example or test created to check this

[yudizhangzyd]

Hi Shahar,
For supporting heterogeneous graphs in GNNExaplainer, the only thing to do is change this function _create_subgraph, right? Thanks.