Add optimizer hooks in callbacks

pytorch_lightning/callbacks/base.py

@@ -166,3 +166,15 @@ def on_save_checkpoint(self, trainer, pl_module):
     def on_load_checkpoint(self, checkpointed_state):
         """Called when loading a model checkpoint, use to reload state."""
         pass
+
+    def on_after_backward(self, trainer, pl_module):
+        """
+        Called after loss.backward() and before optimizers do anything.
+        """
+        pass
+
+    def on_before_zero_grad(self, trainer, pl_module, optimizer):
+        """
+        Called after optimizer.step() and before optimizer.zero_grad().
+        """
+        pass

pytorch_lightning/trainer/callback_hook.py

@@ -209,3 +209,17 @@ def on_load_checkpoint(self, checkpoint):
             if state:
                 state = deepcopy(state)
                 callback.on_load_checkpoint(state)
+
+    def on_after_backward(self):
+        """
+        Called after loss.backward() and before optimizers do anything.
+        """
+        for callback in self.callbacks:
+            callback.on_after_backward(self, self.get_model())
+
+    def on_before_zero_grad(self, optimizer):
+        """
+        Called after optimizer.step() and before optimizer.zero_grad().
+        """
+        for callback in self.callbacks:
+            callback.on_before_zero_grad(self, self.get_model(), optimizer)

pytorch_lightning/trainer/training_loop.py

@@ -454,8 +454,7 @@ def optimizer_step(self, optimizer, opt_idx, batch_idx, train_step_and_backward_
             )
 
     def on_before_zero_grad(self, optimizer):
-        model = self.trainer.get_model()
-        model.on_before_zero_grad(optimizer)
+        self.trainer.call_hook('on_before_zero_grad', optimizer)
 
     def optimizer_zero_grad(self, batch_idx, optimizer, opt_idx):
         self.trainer.accelerator_backend.optimizer_zero_grad(batch_idx, optimizer, opt_idx)

tests/callbacks/test_callbacks.py

@@ -55,6 +55,8 @@ def __init__(self):
             self.on_validation_end_called = False
             self.on_test_start_called = False
             self.on_test_end_called = False
+            self.on_after_backward_called = False
+            self.on_before_zero_grad_called = False
 
         def setup(self, trainer, pl_module, stage: str):
             assert isinstance(trainer, Trainer)
@@ -160,6 +162,14 @@ def on_test_end(self, trainer, pl_module):
             _check_args(trainer, pl_module)
             self.on_test_end_called = True
 
+        def on_after_backward(self, trainer, pl_module):
+            _check_args(trainer, pl_module)
+            self.on_after_backward_called = True
+
+        def on_before_zero_grad(self, trainer, pl_module, optimizer):
+            _check_args(trainer, pl_module)
+            self.on_before_zero_grad_called = True
+
     test_callback = TestCallback()
 
     trainer_options = dict(
@@ -197,6 +207,8 @@ def on_test_end(self, trainer, pl_module):
     assert not test_callback.on_validation_end_called
     assert not test_callback.on_test_start_called
     assert not test_callback.on_test_end_called
+    assert not test_callback.on_after_backward_called
+    assert not test_callback.on_before_zero_grad_called
 
     # fit model
     trainer = Trainer(**trainer_options)
@@ -228,6 +240,8 @@ def on_test_end(self, trainer, pl_module):
     assert not test_callback.on_validation_end_called
     assert not test_callback.on_test_start_called
     assert not test_callback.on_test_end_called
+    assert not test_callback.on_after_backward_called
+    assert not test_callback.on_before_zero_grad_called
 
     trainer.fit(model)
 
@@ -257,6 +271,8 @@ def on_test_end(self, trainer, pl_module):
     assert not test_callback.on_test_batch_end_called
     assert not test_callback.on_test_start_called
     assert not test_callback.on_test_end_called
+    assert test_callback.on_after_backward_called
+    assert test_callback.on_before_zero_grad_called
 
     # reset setup teardown callback
     test_callback.teardown_called = False
@@ -277,3 +293,5 @@ def on_test_end(self, trainer, pl_module):
     assert not test_callback.on_validation_end_called
     assert not test_callback.on_validation_batch_end_called
     assert not test_callback.on_validation_batch_start_called
+    assert not test_callback.on_after_backward_called
+    assert not test_callback.on_before_zero_grad_called