Fix ModelParallelStrategy fails with non-distributed checkpoint.

src/lightning/pytorch/CHANGELOG.md

@@ -6,19 +6,11 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ---
 
-## [Unreleased] - YYYY-MM-DD
-
-### Added
-
--
-
-### Changed
-
--
+### Fixed
 
-### Removed
+- Fixed ``ModelParallelStrategy`` single-file checkpointing when ``torch.compile`` wraps the model so optimizer states no longer raise ``KeyError`` during save ([#21357](https://github.com/Lightning-AI/pytorch-lightning/issues/21357))
 
--
+---
 
 
 ## [2.6.0] - 2025-11-28

src/lightning/pytorch/strategies/model_parallel.py

@@ -286,7 +286,7 @@ def optimizer_state(self, optimizer: Optimizer) -> dict[str, Any]:
 
         state_dict = get_optimizer_state_dict(self.model, optimizer, options=state_dict_options)
         if not self._save_distributed_checkpoint and self.global_rank == 0:
-            # Store the optimizer state dict in standard format
+            state_dict = _align_compiled_param_names_with_module(state_dict, self.model)
             state_dict = FSDP.rekey_optim_state_dict(state_dict, OptimStateKeyType.PARAM_ID, self.model)
         return state_dict
 
@@ -366,3 +366,55 @@ def set_world_ranks(self) -> None:
         # `LightningEnvironment.set_global_rank` will do this too, but we cannot rely on that implementation detail
         # additionally, for some implementations, the setter is a no-op, so it's safer to access the getter
         rank_zero_only.rank = utils_rank_zero_only.rank = self.global_rank
+
+
+def _align_compiled_param_names_with_module(state_dict: dict[str, Any], module: torch.nn.Module) -> dict[str, Any]:
+    """Align optimizer state dict keys with a module that may have compiled submodules.
+
+    When ``torch.compile`` wraps a submodule, its parameters appear under ``_orig_mod``.
+    For example, ``model.0.weight`` becomes ``model._orig_mod.0.weight``.  The optimizer
+    state dict returned by ``get_optimizer_state_dict`` may not include the ``_orig_mod``
+    prefix, causing a mismatch when ``rekey_optim_state_dict`` builds its mapping from
+    ``module.named_parameters()``.
+
+    This function inserts ``._orig_mod`` into the state dict keys where necessary so that
+    they match the module's ``named_parameters()`` output.
+
+    """
+    from torch._dynamo import OptimizedModule
+
+    # Build set of compiled submodule prefixes (e.g., "model" if model is compiled)
+    compiled_prefixes: list[str] = []
+    for name, submodule in module.named_modules():
+        if isinstance(submodule, OptimizedModule):
+            compiled_prefixes.append(name)
+
+    if not compiled_prefixes:
+        return state_dict
+
+    # Sort by length descending so longer prefixes are matched first
+    compiled_prefixes.sort(key=len, reverse=True)
+
+    def _transform_key(key: str) -> str:
+        for prefix in compiled_prefixes:
+            # Check if key starts with "prefix." (the compiled module path)
+            if key == prefix or key.startswith(prefix + "."):
+                suffix = key[len(prefix) :]  # e.g., ".0.weight" or ""
+                # Insert _orig_mod between prefix and rest
+                return f"{prefix}._orig_mod{suffix}"
+        return key
+
+    # Transform keys in "state" section of the optimizer state dict
+    if "state" in state_dict:
+        new_state = {_transform_key(k): v for k, v in state_dict["state"].items()}
+        state_dict = {**state_dict, "state": new_state}
+
+    # Transform param names in "param_groups" section
+    if "param_groups" in state_dict:
+        new_param_groups = []
+        for group in state_dict["param_groups"]:
+            new_group = {**group, "params": [_transform_key(p) for p in group["params"]]}
+            new_param_groups.append(new_group)
+        state_dict = {**state_dict, "param_groups": new_param_groups}
+
+    return state_dict

tests/tests_pytorch/strategies/test_model_parallel.py

@@ -251,3 +251,78 @@ def configure_model(self) -> None:
         strategy.setup(Mock())
     assert all(not p.is_meta for p in model.parameters())
     assert all(not b.is_meta for b in model.buffers())
+
+
+@RunIf(min_torch="2.4")
+def test_align_compiled_param_names_with_module():
+    """Test that optimizer state dict keys are aligned with compiled submodule parameter names."""
+    from lightning.pytorch.strategies.model_parallel import _align_compiled_param_names_with_module
+
+    class SimpleModule(nn.Module):
+        def __init__(self):
+            super().__init__()
+            self.model = nn.Sequential(nn.Linear(32, 64), nn.ReLU(), nn.Linear(64, 32))
+
+        def forward(self, x):
+            return self.model(x)
+
+    # Test with compiled submodule
+    m = SimpleModule()
+    m.model = torch.compile(m.model)
+
+    # Simulate optimizer state dict without _orig_mod in keys (includes both state and param_groups)
+    state_dict = {
+        "state": {
+            "model.0.weight": {"step": 1},
+            "model.0.bias": {"step": 1},
+            "model.2.weight": {"step": 1},
+            "model.2.bias": {"step": 1},
+        },
+        "param_groups": [{"params": ["model.0.weight", "model.0.bias", "model.2.weight", "model.2.bias"], "lr": 0.01}],
+    }
+
+    result = _align_compiled_param_names_with_module(state_dict, m)
+
+    # Verify state keys now have _orig_mod inserted
+    expected_keys = {
+        "model._orig_mod.0.weight",
+        "model._orig_mod.0.bias",
+        "model._orig_mod.2.weight",
+        "model._orig_mod.2.bias",
+    }
+    assert set(result["state"].keys()) == expected_keys
+
+    # Verify param_groups params also have _orig_mod inserted
+    assert set(result["param_groups"][0]["params"]) == expected_keys
+
+    # Verify they match the module's named_parameters
+    param_names = {name for name, _ in m.named_parameters()}
+    assert set(result["state"].keys()) == param_names
+
+
+@RunIf(min_torch="2.4")
+def test_align_compiled_param_names_no_compile():
+    """Test that non-compiled modules pass through unchanged."""
+    from lightning.pytorch.strategies.model_parallel import _align_compiled_param_names_with_module
+
+    class SimpleModule(nn.Module):
+        def __init__(self):
+            super().__init__()
+            self.model = nn.Sequential(nn.Linear(32, 64), nn.Linear(64, 32))
+
+        def forward(self, x):
+            return self.model(x)
+
+    m = SimpleModule()  # Not compiled
+
+    state_dict = {
+        "state": {
+            "model.0.weight": {"step": 1},
+            "model.0.bias": {"step": 1},
+        }
+    }
+
+    result = _align_compiled_param_names_with_module(state_dict, m)
+
+    # Keys should be unchanged
+    assert set(result["state"].keys()) == {"model.0.weight", "model.0.bias"}

tests/tests_pytorch/strategies/test_model_parallel_integration.py

@@ -135,6 +135,33 @@ def configure_model(self):
         parallelize(self.model, device_mesh=self.device_mesh)
 
 
+class SimpleCompiledModule(LightningModule):
+    def __init__(self):
+        super().__init__()
+        self.model = nn.Sequential(nn.Linear(32, 64), nn.ReLU(), nn.Linear(64, 32))
+        self._loss = nn.MSELoss()
+
+    def configure_model(self):
+        self.model = torch.compile(self.model)
+
+    def training_step(self, batch, batch_idx):
+        x, y = batch
+        preds = self.model(x)
+        return self._loss(preds, y)
+
+    def configure_optimizers(self):
+        return torch.optim.AdamW(self.parameters(), lr=1e-3)
+
+
+def _compiled_model_dataloader(batch_size: int = 32, num_batches: int = 2):
+    total_samples = batch_size * num_batches
+    generator = torch.Generator().manual_seed(0)
+    features = torch.randn(total_samples, 32, generator=generator)
+    targets = torch.randn(total_samples, 32, generator=generator)
+    dataset = torch.utils.data.TensorDataset(features, targets)
+    return DataLoader(dataset, batch_size=batch_size)
+
+
 @RunIf(min_torch="2.4", standalone=True, min_cuda_gpus=4)
 def test_setup_device_mesh(distributed):
     from torch.distributed.device_mesh import DeviceMesh
@@ -237,6 +264,44 @@ def training_step(self, batch):
     trainer.fit(model)
 
 
+@RunIf(min_torch="2.4", standalone=True, min_cuda_gpus=2)
+def test_model_parallel_single_file_checkpoint_with_compile(distributed, tmp_path):
+    """Replicate the reporter's setup: compiled model + ModelParallel single-file checkpointing."""
+
+    seed_everything(0)
+    strategy = ModelParallelStrategy(
+        data_parallel_size=1,
+        tensor_parallel_size=1,
+        save_distributed_checkpoint=False,
+    )
+
+    trainer = Trainer(
+        accelerator="auto",
+        devices=1,
+        strategy=strategy,
+        max_steps=2,
+        limit_train_batches=2,
+        enable_checkpointing=False,
+        logger=False,
+        enable_progress_bar=False,
+        enable_model_summary=False,
+        default_root_dir=tmp_path,
+    )
+
+    dataloader = _compiled_model_dataloader(batch_size=32, num_batches=2)
+
+    with trainer.init_module(empty_init=True):
+        model = SimpleCompiledModule()
+
+    trainer.fit(model, dataloader)
+
+    if trainer.is_global_zero:
+        checkpoint_path = tmp_path / "compiled-model.ckpt"
+        trainer.save_checkpoint(checkpoint_path)
+
+    trainer.strategy.barrier()
+
+
 @RunIf(min_torch="2.4", standalone=True, min_cuda_gpus=4)
 @pytest.mark.parametrize(
     "compile",