[bugfix] Check LightningOptimizer doesn't delete optimizer hooks (#6305)

* update

* resolve bug

pytorch_lightning/core/optimizer.py

@@ -38,7 +38,7 @@ class LightningOptimizer:
 
     def __init__(self, optimizer: Optimizer):
 
-        self.__dict__ = {k: v for k, v in optimizer.__dict__.items() if k != 'step'}
+        self.__dict__ = {k: v for k, v in optimizer.__dict__.items() if k not in ('step', "__del__")}
 
         # For Horovod
         if hasattr(optimizer, "skip_synchronize"):

tests/core/test_lightning_optimizer.py

@@ -11,6 +11,8 @@
 # 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 gc
+from typing import Any
 from unittest.mock import DEFAULT, patch
 
 import torch
@@ -303,3 +305,78 @@ def test_lightning_optimizer_automatic_optimization_lbfgs_zero_grad(tmpdir):
     lbfgs = model.optimizers()
     max_iter = lbfgs.param_groups[0]["max_iter"]
     assert zero_grad.call_count == max_iter
+
+
+class OptimizerWithHooks(Optimizer):
+
+    def __init__(self, model):
+        self._fwd_handles = []
+        self._bwd_handles = []
+        self.params = []
+        for _, mod in model.named_modules():
+            mod_class = mod.__class__.__name__
+            if mod_class != 'Linear':
+                continue
+
+            handle = mod.register_forward_pre_hook(self._save_input)  # save the inputs
+            self._fwd_handles.append(handle)  # collect forward-save-input hooks in list
+            handle = mod.register_backward_hook(self._save_grad_output)  # save the gradients
+            self._bwd_handles.append(handle)  # collect backward-save-grad hook in list
+
+            # save the parameters
+            params = [mod.weight]
+            if mod.bias is not None:
+                params.append(mod.bias)
+
+            # save a param_group for each module
+            d = {'params': params, 'mod': mod, 'layer_type': mod_class}
+            self.params.append(d)
+
+        super(OptimizerWithHooks, self).__init__(self.params, {"lr": 0.01})
+
+    def _save_input(self, mod, i):
+        """Saves input of layer"""
+        if mod.training:
+            self.state[mod]['x'] = i[0]
+
+    def _save_grad_output(self, mod, _, grad_output):
+        """
+        Saves grad on output of layer to
+        grad is scaled with batch_size since gradient is spread over samples in mini batch
+        """
+        batch_size = grad_output[0].shape[0]
+        if mod.training:
+            self.state[mod]['grad'] = grad_output[0] * batch_size
+
+    def step(self, closure=None):
+        closure()
+        for group in self.param_groups:
+            _ = self.state[group['mod']]['x']
+            _ = self.state[group['mod']]['grad']
+        return True
+
+
+def test_lightning_optimizer_keeps_hooks(tmpdir):
+
+    class TestModel(BoringModel):
+        count_on_train_batch_start = 0
+        count_on_train_batch_end = 0
+
+        def configure_optimizers(self):
+            return OptimizerWithHooks(self)
+
+        def on_train_batch_start(self, batch: Any, batch_idx: int, dataloader_idx: int) -> None:
+            self.count_on_train_batch_start += 1
+            optimizer = self.optimizers(use_pl_optimizer=False)
+            assert len(optimizer._fwd_handles) == 1
+
+        def on_train_batch_end(self, outputs: Any, batch: Any, batch_idx: int, dataloader_idx: int) -> None:
+            self.count_on_train_batch_end += 1
+            del self.trainer._lightning_optimizers
+            gc.collect()  # not necessary, just in case
+
+    trainer = Trainer(default_root_dir=tmpdir, limit_train_batches=4, limit_val_batches=1, max_epochs=1)
+    model = TestModel()
+    trainer.fit(model)
+    assert model.count_on_train_batch_start == 4
+    assert model.count_on_train_batch_end == 4