made ddp the default if no backend specified with multiple GPUs (#1789)

* made ddp the default if no backend specified with multiple GPUs

* fix

* spawn

Co-authored-by: Jirka <jirka.borovec@seznam.cz>

tests/models/test_gpu.py

@@ -130,6 +130,7 @@ def test_cpu_slurm_save_load(tmpdir):
     trainer.fit(model)
 
 
+@pytest.mark.spawn
 @pytest.mark.skipif(torch.cuda.device_count() < 2, reason="test requires multi-GPU machine")
 def test_multi_gpu_none_backend(tmpdir):
     """Make sure when using multiple GPUs the user can't use `distributed_backend = None`."""

tests/trainer/test_trainer.py

@@ -712,7 +712,7 @@ def test_gpu_choice(tmpdir):
     ),
     pytest.param(
         dict(distributed_backend=None, gpus=2),
-        dict(use_dp=True, use_ddp=False, use_ddp2=False, num_gpus=2, on_gpu=True, single_gpu=False, num_processes=1),
+        dict(use_dp=False, use_ddp=True, use_ddp2=False, num_gpus=2, on_gpu=True, single_gpu=False, num_processes=1),
         marks=[pytest.mark.skipif(torch.cuda.device_count() < 2, reason="Multiple GPUs needed")]
     ),
     pytest.param(