[bugfix] Fix crash due to unfound attribute (#2262)

An unexpected `AttributeError` exception is launched whenever the user defines `save_dir` on `DGLDataset` instantiation and uses `self._save_dir`  within `has_cache` method.
This commit fixes the issue.

python/dgl/data/dgl_dataset.py

@@ -87,6 +87,8 @@ class DGLDataset(object):
 
         if save_dir is None:
             self._save_dir = self._raw_dir
+        else:
+            self._save_dir = save_dir
 
         self._load()