[bugfix] Fix set_default_backend() keyword (#3710)

* Add unit test

* Fix typo
Co-authored-by: Jinjing Zhou <VoVAllen@users.noreply.github.com>

python/dgl/backend/set_default_backend.py

@@ -3,8 +3,7 @@ import os
 import json
 
 def set_default_backend(default_dir, backend_name):
-    # the exists_ok requires python >= 3.2
-    os.makedirs(default_dir, exists_ok=True)
+    os.makedirs(default_dir, exist_ok=True)
     config_path = os.path.join(default_dir, 'config.json')
     with open(config_path, "w") as config_file: 
         json.dump({'backend': backend_name.lower()}, config_file)

tests/compute/test_backend.py

+import backend as F
+import os
+import unittest
+
+
+def test_set_default_backend():
+    default_dir = os.path.join(os.path.expanduser('~'), '.dgl_unit_test')
+    F.set_default_backend(default_dir, 'pytorch')
+
+    # make sure the config file was created
+    assert os.path.exists(os.path.join(default_dir, 'config.json'))