add pl_multi_process_test decorator to TPU num_devices test

tests/accelerators/test_tpu.py

@@ -94,6 +94,7 @@ def test_accelerator_cpu_with_tpu_cores_flag():
 
 
 @RunIf(tpu=True)
+@pl_multi_process_test
 @pytest.mark.parametrize(["accelerator", "devices"], [("auto", 8), ("auto", "auto"), ("tpu", None)])
 def test_accelerator_tpu(accelerator, devices):
     assert TPUAccelerator.is_available()
@@ -117,8 +118,8 @@ def test_accelerator_tpu_with_tpu_cores_priority():
 
 
 @RunIf(tpu=True)
+@pl_multi_process_test
 def test_set_devices_if_none_tpu():
-
     trainer = Trainer(accelerator="tpu", tpu_cores=8)
     assert trainer.num_devices == 8
 
@@ -313,6 +314,7 @@ def test_warning_if_tpus_not_used():
 
 
 @RunIf(tpu=True)
+@pl_multi_process_test
 @pytest.mark.parametrize(
     ["devices", "expected_device_ids"],
     [