fix: nb is set total number of devices, when nb is -1.

docs/source/multi_gpu.rst

@@ -206,6 +206,8 @@ Note in particular the difference between `gpus=0`, `gpus=[0]` and `gpus="0"`.
     `auto_select_gpus=True` will automatically help you find `k` gpus that are not
     occupied by other processes. This is especially useful when GPUs are configured
     to be in "exclusive mode", such that only one process at a time can access them.
+    For more details see the :ref:`Trainer guide <trainer>`.
+
 
 Remove CUDA flags
 ^^^^^^^^^^^^^^^^^

pytorch_lightning/trainer/__init__.py

@@ -381,6 +381,12 @@ def training_step(self, batch, batch_idx, optimizer_idx):
     # enable auto selection (will find two available gpus on system)
     trainer = Trainer(gpus=2, auto_select_gpus=True)
 
+    # specifies all GPUs regardless of its availability
+    Trainer(gpus=-1, auto_select_gpus=False)
+
+    # specifies all available GPUs (if only one GPU is not occupied, uses one gpu)
+    Trainer(gpus=-1, auto_select_gpus=True)
+
 auto_lr_find
 ^^^^^^^^^^^^
 

pytorch_lightning/tuner/auto_gpu_select.py

@@ -13,8 +13,18 @@
 # limitations under the License.
 import torch
 
+from pytorch_lightning.utilities.exceptions import MisconfigurationException
+
 
 def pick_multiple_gpus(nb):
+    if nb == 0:
+        raise MisconfigurationException(
+            r"auto_select_gpus=True, gpus=0 is not a valid configuration.\
+            Please select a valid number of GPU resources when using auto_select_gpus."
+        )
+
+    nb = torch.cuda.device_count() if nb == -1 else nb
+
     picked = []
     for _ in range(nb):
         picked.append(pick_single_gpu(exclude_gpus=picked))

tests/tuner/test_auto_gpu_select.py

@@ -0,0 +1,74 @@
+# Copyright The PyTorch Lightning team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import re
+
+import pytest
+import torch
+
+from pytorch_lightning import Trainer
+from pytorch_lightning.tuner.auto_gpu_select import pick_multiple_gpus
+from pytorch_lightning.utilities.exceptions import MisconfigurationException
+
+
+@pytest.mark.skipif(
+    torch.cuda.device_count() < 2, reason="test requires a number of GPU machine greater than 1"
+)
+@pytest.mark.parametrize(
+    ["auto_select_gpus", "gpus", "expected_error"],
+    [
+        (True, 0, MisconfigurationException),
+        (True, -1, None),
+        (False, 0, None),
+        (False, -1, None),
+    ],
+)
+def test_trainer_with_gpus_options_combination_at_available_gpus_env(
+    auto_select_gpus, gpus, expected_error
+):
+    if expected_error:
+        with pytest.raises(
+            expected_error,
+            match=re.escape(
+                r"auto_select_gpus=True, gpus=0 is not a valid configuration.\
+            Please select a valid number of GPU resources when using auto_select_gpus."
+            ),
+        ):
+            trainer = Trainer(auto_select_gpus=auto_select_gpus, gpus=gpus)
+    else:
+        trainer = Trainer(auto_select_gpus=auto_select_gpus, gpus=gpus)
+
+
+@pytest.mark.skipif(
+    torch.cuda.device_count() < 2, reason="test requires a number of GPU machine greater than 1"
+)
+@pytest.mark.parametrize(
+    ["nb", "expected_gpu_idxs", "expected_error"],
+    [
+        (0, [], MisconfigurationException),
+        (-1, [i for i in range(torch.cuda.device_count())], None),
+        (1, [0], None),
+    ],
+)
+def test_pick_multiple_gpus(nb, expected_gpu_idxs, expected_error):
+    if expected_error:
+        with pytest.raises(
+            expected_error,
+            match=re.escape(
+                r"auto_select_gpus=True, gpus=0 is not a valid configuration.\
+            Please select a valid number of GPU resources when using auto_select_gpus."
+            ),
+        ):
+            pick_multiple_gpus(nb)
+    else:
+        assert expected_gpu_idxs == pick_multiple_gpus(nb)