Document ModelCheckpoint.file_exists DDP memory fix

Author: littlebullGit

src/lightning/pytorch/CHANGELOG.md

@@ -82,6 +82,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed FSDP mixed precision semantics and added user warning ([#21361](https://github.com/Lightning-AI/pytorch-lightning/pull/21361))
 
 
+- Fixed `ModelCheckpoint.file_exists` using broadcast in DDP, reducing memory usage when checking for existing checkpoints ([#19674](https://github.com/Lightning-AI/pytorch-lightning/issues/19674))
+
+
 ---
 
 ## [2.5.6] - 2025-11-05

tests/tests_pytorch/checkpointing/test_model_checkpoint.py

@@ -2180,36 +2180,3 @@ def on_validation_epoch_end(self):
     assert len(checkpoint_files) == expected_files, (
         f"Expected {expected_files} files, got {len(checkpoint_files)}: {checkpoint_names}"
     )
-
-
-def test_model_checkpoint_file_exists_distributed_branch(tmp_path):
-    """Ensure the distributed branch of ModelCheckpoint.file_exists uses reduce_boolean_decision."""
-
-    checkpoint = ModelCheckpoint(dirpath=tmp_path)
-    calls = []
-
-    class DummyStrategy:
-        def reduce_boolean_decision(self, decision, all=True):
-            calls.append((decision, all))
-            return decision
-
-    class DummyTrainer:
-        def __init__(self, is_global_zero: bool):
-            self.world_size = 2
-            self.is_global_zero = is_global_zero
-            self.strategy = DummyStrategy()
-
-    # global rank 0: filesystem is touched and decision=True is reduced with all=False
-    checkpoint._fs.exists = Mock(return_value=True)
-    trainer = DummyTrainer(is_global_zero=True)
-    assert checkpoint.file_exists("ignored", trainer)
-    checkpoint._fs.exists.assert_called_once_with("ignored")
-    assert calls == [(True, False)]
-
-    # non-global ranks: filesystem is not touched and local decision is False
-    calls.clear()
-    checkpoint._fs.exists = Mock(return_value=True)
-    trainer = DummyTrainer(is_global_zero=False)
-    assert not checkpoint.file_exists("ignored", trainer)
-    checkpoint._fs.exists.assert_not_called()
-    assert calls == [(False, False)]