Fix failing GPU tests (Lightning-AI#722)

* Fix distributed_backend=None test

We now throw a warning instead of an exception. Update test
to reflect this.

* Fix test_tube logger close when debug=True

Author: neggert

pytorch_lightning/logging/test_tube.py

@@ -135,8 +135,9 @@ def finalize(self, status):
     def close(self):
         # TODO: HACK figure out where this is being set to true
         self.experiment.debug = self.debug
-        exp = self.experiment
-        exp.close()
+        if not self.debug:
+            exp = self.experiment
+            exp.close()
 
     @property
     def rank(self):

tests/test_gpu_models.py

@@ -183,7 +183,7 @@ def test_multi_gpu_none_backend(tmpdir):
         gpus='-1'
     )
 
-    with pytest.raises(MisconfigurationException):
+    with pytest.warns(UserWarning):
         tutils.run_model_test(trainer_options, model)