Fix directed/undirected pre-process option in CitationFull (#8179)

Fixes #8171

Author: rusty1s

Diff for: torch_geometric/datasets/citation_full.py

@@ -98,7 +98,8 @@ def raw_file_names(self) -> str:
 
     @property
     def processed_file_names(self) -> str:
-        return 'data.pt'
+        suffix = 'undirected' if self.to_undirected else 'directed'
+        return f'data_{suffix}.pt'
 
     def download(self):
         download_url(self.url.format(self.name), self.raw_dir)