Synchronize gradients in manual optimization with DDPStrategy(static_graph=True) (#21251)

* fix: synchronize gradients in manual optimization with DDPStrategy(static_graph=True). Ensure gradients are reduced correctly when using manual optimization and DDP with static_graph enabled.

* Adds regression test to cover all combinations of optimization/static_graph.

* Initialize _pl_static_graph_delay_done attribute properly

* changelog

---------

Co-authored-by: Nicki Skafte Detlefsen <skaftenicki@gmail.com>

Author: Sohaib-Ahmed21

src/lightning/pytorch/CHANGELOG.md

@@ -61,6 +61,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed how `ThroughputMonitor` calculated training time ([#21291](https://github.com/Lightning-AI/pytorch-lightning/pull/21291))
 
 
+- Fixed synchronization of gradients in manual optimization with `DDPStrategy(static_graph=True)` ([#21251](https://github.com/Lightning-AI/pytorch-lightning/pull/21251))
+
+
+
 ---
 
 ## [2.5.5] - 2025-09-05

src/lightning/pytorch/strategies/ddp.py

@@ -103,6 +103,7 @@ def __init__(
         self._process_group_backend: Optional[str] = process_group_backend
         self._timeout: Optional[timedelta] = timeout
         self._start_method = start_method
+        self._pl_static_graph_delay_done = False
 
     @property
     def is_distributed(self) -> bool:  # pragma: no-cover
@@ -319,6 +320,27 @@ def pre_backward(self, closure_loss: Tensor) -> None:
         if not self.lightning_module.automatic_optimization:
             prepare_for_backward(self.model, closure_loss)
 
+    @override
+    def post_backward(self, closure_loss: Tensor) -> None:
+        # Only for first static-graph iteration with manual optimization
+        model = self.model
+        lm = self.lightning_module
+        if not isinstance(model, DistributedDataParallel):
+            return
+        if lm is None or lm.automatic_optimization:
+            return
+        if not getattr(model, "static_graph", False):
+            return
+        if self._pl_static_graph_delay_done:
+            return
+
+        # Call DDP's own first-iter static-graph flush.
+        # This is what actually launches the bucket all-reduces.
+        reducer = model.reducer
+        reducer._delay_all_reduce()
+
+        self._pl_static_graph_delay_done = True
+
     @override
     def model_to_device(self) -> None:
         log.debug(f"{self.__class__.__name__}: moving model to device [{self.root_device}]...")

tests/tests_pytorch/strategies/test_ddp_integration.py

@@ -448,3 +448,50 @@ def creates_processes_externally(self):
         RuntimeError, match="Lightning attempted to launch new distributed processes with `local_rank > 0`."
     ):
         trainer.fit(model)
+
+
+@RunIf(min_cuda_gpus=2, standalone=True)
+@pytest.mark.parametrize("automatic_optimization", [True, False])
+@pytest.mark.parametrize("static_graph", [True, False])
+def test_ddp_gradients_synced(tmp_path, automatic_optimization, static_graph):
+    """Ensure gradients are synchronized across ranks for both optimization modes and static_graph settings."""
+
+    class TestModel(BoringModel):
+        def __init__(self):
+            super().__init__()
+            self.automatic_optimization = automatic_optimization
+
+        def training_step(self, batch, batch_idx):
+            if self.automatic_optimization:
+                return super().training_step(batch, batch_idx)
+
+            # manual optimization path
+            opt = self.optimizers()
+            opt.zero_grad()
+            out = super().training_step(batch, batch_idx)
+            loss = out["loss"]
+            self.manual_backward(loss)
+            opt.step()
+            return out
+
+        def on_train_batch_end(self, *args, **kwargs):
+            # record grad sum for sync check
+            grad_sum = self.layer.bias.grad.detach().sum()
+            self.log("grad_sum_min", grad_sum, sync_dist=True, reduce_fx="min")
+            self.log("grad_sum_max", grad_sum, sync_dist=True, reduce_fx="max")
+
+    trainer = Trainer(
+        default_root_dir=tmp_path,
+        accelerator="gpu",
+        devices=2,
+        strategy=DDPStrategy(static_graph=static_graph),
+        max_steps=1,
+        enable_progress_bar=False,
+        enable_model_summary=False,
+    )
+    trainer.fit(TestModel(), datamodule=BoringDataModule())
+
+    # assert all ranks saw identical grads
+    gmin = trainer.callback_metrics["grad_sum_min"]
+    gmax = trainer.callback_metrics["grad_sum_max"]
+    assert torch.allclose(gmin, gmax)