[doc] add booster docstring and fix autodoc (#3789)

* [doc] add docstr for booster methods

* [doc] fix autodoc

colossalai/booster/booster.py

@@ -130,6 +130,12 @@ class Booster:
         return model, optimizer, criterion, dataloader, lr_scheduler
 
     def backward(self, loss: torch.Tensor, optimizer: Optimizer) -> None:
+        """Backward pass.
+
+        Args:
+            loss (torch.Tensor): The loss to be backpropagated.
+            optimizer (Optimizer): The optimizer to be updated.
+        """
         # TODO: implement this method with plugin
         optimizer.backward(loss)
 
@@ -146,6 +152,14 @@ class Booster:
         pass
 
     def no_sync(self, model: nn.Module) -> contextmanager:
+        """Context manager to disable gradient synchronization across DP process groups.
+
+        Args:
+            model (nn.Module): The model to be disabled gradient synchronization.
+
+        Returns:
+            contextmanager: Context to disable gradient synchronization.
+        """
         assert self.plugin is not None, f'no_sync is only enabled when a plugin is provided and the plugin supports no_sync.'
         assert self.plugin.support_no_sync, f'The plugin {self.plugin.__class__.__name__} does not support no_sync.'
         return self.plugin.no_sync(model)