split `restore_training_state` into logical parts [1 / 2] (#7901)

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

pytorch_lightning/trainer/connectors/checkpoint_connector.py

@@ -207,6 +207,92 @@ class CheckpointConnector:
         for scheduler, lrs_state in zip(self.trainer.lr_schedulers, lr_schedulers):
             scheduler['scheduler'].load_state_dict(lrs_state)
 
+    def restore_callbacks(self) -> None:
+        """ Restores all callbacks from the pre-loaded checkpoint. """
+        if not self._loaded_checkpoint:
+            return
+
+        if any(key in self._loaded_checkpoint for key in DEPRECATED_CHECKPOINT_KEYS):
+            raise ValueError(
+                "The checkpoint you're attempting to load follows an"
+                " outdated schema. You can upgrade to the current schema by running"
+                " `python -m pytorch_lightning.utilities.upgrade_checkpoint --file model.ckpt`"
+                " where `model.ckpt` is your checkpoint file."
+            )
+        self.trainer.on_load_checkpoint(self._loaded_checkpoint)
+
+    def restore_progress(self) -> None:
+        """
+        Restores the training progress from the pre-loaded checkpoint. This currently includes only the global step
+        and current epoch.
+        """
+        if not self._loaded_checkpoint:
+            return
+
+        self.trainer.train_loop.global_step = self._loaded_checkpoint['global_step']
+        self.trainer.train_loop.current_epoch = self._loaded_checkpoint['epoch']
+
+        # crash if max_epochs is lower then the current epoch from the checkpoint
+        if self.trainer.max_epochs is not None and self.trainer.current_epoch > self.trainer.max_epochs:
+            raise MisconfigurationException(
+                f"You restored a checkpoint with current_epoch={self.trainer.current_epoch},"
+                f" but you have set Trainer(max_epochs={self.trainer.max_epochs})."
+            )
+
+        # Division deals with global step stepping once per accumulated batch
+        # Inequality deals with different global step for odd vs even num_training_batches
+        n_accum = 1 if self.trainer.accumulate_grad_batches is None else self.trainer.accumulate_grad_batches
+        expected_steps = self.trainer.num_training_batches / n_accum
+        if self.trainer.num_training_batches != 0 and self.trainer.global_step % expected_steps > 1:
+            rank_zero_warn(
+                "You're resuming from a checkpoint that ended mid-epoch."
+                " Training will start from the beginning of the next epoch."
+                " This can cause unreliable results if further training is done,"
+                " consider using an end of epoch checkpoint."
+            )
+
+    def restore_optimizers_and_schedulers(self) -> None:
+        """ Restores the optimizers and learning rate scheduler states from the pre-loaded checkpoint. """
+        if not self._load_optimizer_states or not self._loaded_checkpoint:
+            return
+
+        # validation
+        if "optimizer_states" not in self._loaded_checkpoint or "lr_schedulers" not in self._loaded_checkpoint:
+            raise KeyError(
+                "Trying to restore training state but checkpoint contains only the model."
+                " This is probably due to `ModelCheckpoint.save_weights_only` being set to `True`."
+            )
+        self.restore_optimizers()
+        self.restore_lr_schedulers()
+
+    def restore_optimizers(self) -> None:
+        """ Restores the optimizer states from the pre-loaded checkpoint. """
+        if not self._load_optimizer_states or not self._loaded_checkpoint:
+            return
+
+        # restore the optimizers
+        optimizer_states = self._loaded_checkpoint['optimizer_states']
+        for optimizer, opt_state in zip(self.trainer.optimizers, optimizer_states):
+            optimizer.load_state_dict(opt_state)
+
+            # move optimizer to GPU 1 weight at a time
+            # avoids OOM
+            if self.trainer.root_gpu is not None:
+                for state in optimizer.state.values():
+                    for k, v in state.items():
+                        if isinstance(v, torch.Tensor):
+                            state[k] = v.cuda(self.trainer.root_gpu)
+
+    def restore_lr_schedulers(self) -> None:
+        """ Restores the learning rate scheduler states from the pre-loaded checkpoint. """
+        if not self._load_optimizer_states or not self._loaded_checkpoint:
+            return
+
+        # restore the lr schedulers
+        lr_schedulers = self._loaded_checkpoint['lr_schedulers']
+        for scheduler, lrs_state in zip(self.trainer.lr_schedulers, lr_schedulers):
+            scheduler['scheduler'].load_state_dict(lrs_state)
+
     # ----------------------------------
     # PRIVATE OPS
     # ----------------------------------