Fix ReduceOp type hint in ColossalAI strategy (#15535)

(cherry picked from commit 62d040c)

Author: awaelchli

src/pytorch_lightning/CHANGELOG.md

@@ -48,9 +48,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Fixed `TensorBoardLogger` not validating the input array type when logging the model graph ([#15323](https://github.com/Lightning-AI/lightning/pull/15323))
 
--
-
--
+- Fixed an attribute error in `ColossalAIStrategy` at import time when `torch.distributed` is not available ([#15535](https://github.com/Lightning-AI/lightning/pull/15535))
 
 - Fixed an issue with the `BaseFinetuning` callback not setting the `track_running_stats` attribute for batch normaliztion layers ([#15063](https://github.com/Lightning-AI/lightning/pull/15063))
 

src/pytorch_lightning/strategies/colossalai.py

@@ -432,7 +432,7 @@ def register_strategies(cls, strategy_registry: Dict) -> None:
         strategy_registry.register("colossalai", cls, description="Default ColossalAI Strategy")
 
     def reduce(
-        self, tensor: Tensor, group: Optional[Any] = None, reduce_op: Optional[Union[ReduceOp, str]] = ReduceOp.SUM
+        self, tensor: Tensor, group: Optional[Any] = None, reduce_op: Optional[Union[ReduceOp, str]] = "sum"
     ) -> Tensor:
         with _patch_cuda_is_available():
             from colossalai.communication.collective import reduce

tests/tests_lite/utilities/test_imports.py

@@ -11,6 +11,10 @@
 # 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 subprocess
+import sys
+from textwrap import dedent
+
 from lightning_lite.strategies.deepspeed import _DEEPSPEED_AVAILABLE
 from lightning_lite.strategies.fairscale import _FAIRSCALE_AVAILABLE
 
@@ -29,3 +33,16 @@ def test_imports():
         assert not _FAIRSCALE_AVAILABLE
     else:
         assert _FAIRSCALE_AVAILABLE
+
+
+def test_import_lightning_lite_with_torch_dist_unavailable():
+    """Test that the package can be imported regardless of whether torch.distributed is available."""
+    code = dedent(
+        """
+        import torch
+        torch.distributed.is_available = lambda: False  # pretend torch.distributed not available
+        import lightning_lite
+        """
+    )
+    # run in complete isolation
+    assert subprocess.call([sys.executable, "-c", code]) == 0

tests/tests_pytorch/utilities/test_imports.py

@@ -14,6 +14,9 @@
 
 import importlib
 import operator
+import subprocess
+import sys
+from textwrap import dedent
 from unittest import mock
 
 import pytest
@@ -141,3 +144,16 @@ def test_import_with_unavailable_dependencies(patch_name, new_fn, to_import, cle
     """
     with mock.patch(patch_name, new=new_fn):
         importlib.import_module(to_import)
+
+
+def test_import_pytorch_lightning_with_torch_dist_unavailable():
+    """Test that the package can be imported regardless of whether torch.distributed is available."""
+    code = dedent(
+        """
+        import torch
+        torch.distributed.is_available = lambda: False  # pretend torch.distributed not available
+        import pytorch_lightning
+        """
+    )
+    # run in complete isolation
+    assert subprocess.call([sys.executable, "-c", code]) == 0