fix ddp init twice in oss test

Summary:
Pull Request resolved: https://github.com/facebookresearch/d2go/pull/192

Nowadays lightning will initialize process group when using ddp strategy, since `TestLightningTrainNet` does a training with ddp strategy (https://fburl.com/code/a9yp0kzy), the process group ended up initialized after running the test. However there're other tests that will also set up ddp and thus expect non-initialized process group, this is not a problem on sandcastle since the tests run separately, however in OSS env, the tests are running together, so the error happens (eg. https://github.com/facebookresearch/d2go/runs/5668912203?check_suite_focus=true).

This diff adds a clean up step in `TestLightningTrainNet`.

Reviewed By: tglik

Differential Revision: D35099944

fbshipit-source-id: f5b42b2a87d4efd9aa0ed97e6bd2140d80ab9522

.github/workflows/unit_test.yml

@@ -29,4 +29,4 @@ jobs:
         env:
             OSSRUN: 1
         run: |
-          python -m unittest discover tests
+          python -m unittest discover -v -s tests

tests/misc/test_lightning_train_net.py

@@ -5,6 +5,7 @@ import os
 import unittest
 
 import numpy as np
+import torch.distributed as dist
 from d2go.config import CfgNode
 from d2go.config.utils import flatten_config_dict
 from d2go.runner.lightning_task import GeneralizedRCNNTask
@@ -56,3 +57,7 @@ class TestLightningTrainNet(unittest.TestCase):
         accuracy2 = flatten_config_dict(out2.accuracy)
         for k in accuracy:
             np.testing.assert_equal(accuracy[k], accuracy2[k])
+
+    def tearDown(self):
+        if dist.is_initialized():
+            dist.destroy_process_group()