Set devices to 1 when it's just Trainer(accelerator='auto')

pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -472,27 +472,27 @@ def _set_devices_to_cpu_num_processes(self) -> None:
             self._map_devices_to_accelerator(DeviceType.CPU)
 
     def _map_devices_to_accelerator(self, accelerator: str) -> bool:
-        if self.devices is None:
-            return False
         if accelerator == DeviceType.TPU and _TPU_AVAILABLE:
             if self.devices == "auto":
                 self.devices = TPUAccelerator.auto_device_count()
+            self.devices = self.devices or 1
             self.tpu_cores = device_parser.parse_tpu_cores(self.devices)
             return True
         if accelerator == DeviceType.IPU and _IPU_AVAILABLE:
             if self.devices == "auto":
                 self.devices = IPUAccelerator.auto_device_count()
-            self.ipus = self.devices
+            self.ipus = self.devices = self.devices or 1
             return True
         if accelerator == DeviceType.GPU and torch.cuda.is_available():
             if self.devices == "auto":
                 self.devices = GPUAccelerator.auto_device_count()
-            self.gpus = self.devices
+            self.gpus = self.devices = self.devices or 1
             self.parallel_device_ids = device_parser.parse_gpu_ids(self.devices)
             return True
         if accelerator == DeviceType.CPU:
             if self.devices == "auto":
                 self.devices = CPUAccelerator.auto_device_count()
+            self.devices = self.devices or 1
             if not isinstance(self.devices, int):
                 raise MisconfigurationException(
                     "The flag `devices` must be an int with `accelerator='cpu'`,"

tests/accelerators/test_accelerator_connector.py

@@ -1026,6 +1026,23 @@ def test_unsupported_ipu_choice(monkeypatch):
         Trainer(accelerator="ipu", precision=64)
 
 
+@mock.patch("torch.cuda.is_available", return_value=False)
+@mock.patch("pytorch_lightning.utilities.imports._TPU_AVAILABLE", return_value=False)
+@mock.patch("pytorch_lightning.utilities.imports._IPU_AVAILABLE", return_value=False)
+def test_accelerator_auto_choice_and_devices_cpu(is_ipu_available_mock, is_tpu_available_mock, is_gpu_available_mock):
+    trainer = Trainer(accelerator="auto")
+    assert trainer.devices == 1
+    assert trainer.num_processes == 1
+
+
+@mock.patch("torch.cuda.is_available", return_value=True)
+@mock.patch("torch.cuda.device_count", return_value=2)
+def test_accelerator_auto_choice_and_devices_gpu(is_gpu_available_mock, device_count_mock):
+    trainer = Trainer(accelerator="auto")
+    assert trainer.devices == 1
+    assert trainer.gpus == 1
+
+
 @mock.patch("torch.cuda.is_available", return_value=False)
 @mock.patch("pytorch_lightning.utilities.imports._TPU_AVAILABLE", return_value=False)
 @mock.patch("pytorch_lightning.utilities.imports._IPU_AVAILABLE", return_value=False)

tests/accelerators/test_ipu.py

@@ -604,3 +604,10 @@ def test_devices_auto_choice_ipu():
     trainer = Trainer(accelerator="auto", devices="auto")
     assert trainer.devices == 4
     assert trainer.ipus == 4
+
+
+@RunIf(ipu=True)
+def test_accelerator_auto_choice_and_devices_ipu():
+    trainer = Trainer(accelerator="auto")
+    assert trainer.devices == 1
+    assert trainer.ipus == 1

tests/accelerators/test_tpu.py

@@ -317,3 +317,10 @@ def test_devices_auto_choice_tpu():
     trainer = Trainer(accelerator="auto", devices="auto")
     assert trainer.devices == 8
     assert trainer.tpu_cores == 8
+
+
+@RunIf(tpu=True)
+def test_accelerator_auto_choice_and_devices_tpu():
+    trainer = Trainer(accelerator="auto")
+    assert trainer.devices == 1
+    assert trainer.tpu_cores == 1