Proposal(CLI): after_instantiate_classes hook (#20401)

feat(cli): after_instantiate_classes hook

Co-authored-by: Luca Antiga <[email protected]>

src/lightning/pytorch/cli.py

@@ -390,6 +390,7 @@ def __init__(
         self._add_instantiators()
         self.before_instantiate_classes()
         self.instantiate_classes()
+        self.after_instantiate_classes()
 
         if self.subcommand is not None:
             self._run_subcommand(self.subcommand)
@@ -561,6 +562,9 @@ def instantiate_classes(self) -> None:
         self._add_configure_optimizers_method_to_model(self.subcommand)
         self.trainer = self.instantiate_trainer()
 
+    def after_instantiate_classes(self) -> None:
+        """Implement to run some code after instantiating the classes."""
+
     def instantiate_trainer(self, **kwargs: Any) -> Trainer:
         """Instantiates the trainer.