remove hash_func from dataset object (#2572)

Co-authored-by: Minjie Wang <wmjlyjemaine@gmail.com>

python/dgl/data/dgl_dataset.py

@@ -77,7 +77,6 @@ class DGLDataset(object):
         self._force_reload = force_reload
         self._verbose = verbose
         self._hash_key = hash_key
-        self._hash_func = hashlib.sha1()
         self._hash = self._get_hash()
 
         # if no dir is provided, the default dgl download dir is used.
@@ -192,8 +191,9 @@ class DGLDataset(object):
         >>> hash_value
         'a770b222'
         """
-        self._hash_func.update(str(self._hash_key).encode('utf-8'))
-        return self._hash_func.hexdigest()[:8]
+        hash_func = hashlib.sha1()
+        hash_func.update(str(self._hash_key).encode('utf-8'))
+        return hash_func.hexdigest()[:8]
 
     @property
     def url(self):