Fix hanging metrics tests (#5134)

Author: tadejsv

tests/metrics/regression/test_ssim.py

@@ -53,9 +53,7 @@ def _sk_metric(preds, target, data_range, multichannel):
 class TestSSIM(MetricTester):
     atol = 6e-5
 
-    # TODO: for some reason this test hangs with ddp=True
-    # @pytest.mark.parametrize("ddp", [True, False])
-    @pytest.mark.parametrize("ddp", [False])
+    @pytest.mark.parametrize("ddp", [True, False])
     @pytest.mark.parametrize("dist_sync_on_step", [True, False])
     def test_ssim(self, preds, target, multichannel, ddp, dist_sync_on_step):
         self.run_class_metric_test(

tests/metrics/utils.py

@@ -11,6 +11,11 @@
 
 from pytorch_lightning.metrics import Metric
 
+try:
+    set_start_method("spawn")
+except RuntimeError:
+    pass
+
 NUM_PROCESSES = 2
 NUM_BATCHES = 10
 BATCH_SIZE = 32
@@ -165,10 +170,7 @@ def setup_class(self):
         """Setup the metric class. This will spawn the pool of workers that are
         used for metric testing and setup_ddp
         """
-        try:
-            set_start_method("spawn")
-        except RuntimeError:
-            pass
+
         self.poolSize = NUM_PROCESSES
         self.pool = Pool(processes=self.poolSize)
         self.pool.starmap(setup_ddp, [(rank, self.poolSize) for rank in range(self.poolSize)])