refactor: rename optimizer step callback hooks to before and after

- rename on_optimizer_step to before_optimizer_step across the callback protocol, built-in callbacks, and trainer call site
- rename on_after_optimizer_step to after_optimizer_step for the symmetric post-step hook
- document the hook pair and the checkpoint save location in developer and training guides

astrai/trainer/trainer.py

@@ -93,7 +93,7 @@ class Trainer:
                         self._call_callbacks("on_batch_end", context)
 
                         if executor.sync_gradients:
-                            self._call_callbacks("on_optimizer_step", context)
+                            self._call_callbacks("before_optimizer_step", context)
                             context.optimizer.step()
                             context.strategy.on_optimizer_step()
                             context.optimizer.zero_grad()
@@ -101,7 +101,7 @@ class Trainer:
                             if context.scheduler:
                                 context.scheduler.step()
 
-                            self._call_callbacks("on_after_optimizer_step", context)
+                            self._call_callbacks("after_optimizer_step", context)
 
                 self._call_callbacks("on_epoch_end", context)