[legacy] move engine to legacy (#4560)

* [legacy] move engine to legacy

* [example] fix seq parallel example

* [example] fix seq parallel example

* [test] test gemini pluging hang

* [test] test gemini pluging hang

* [test] test gemini pluging hang

* [test] test gemini pluging hang

* [test] test gemini pluging hang

* [example] update seq parallel requirements

colossalai/legacy/engine/gradient_handler/_base_gradient_handler.py

@@ -0,0 +1,25 @@
+#!/usr/bin/env python
+# -*- encoding: utf-8 -*-
+
+from abc import ABC, abstractmethod
+
+
+class BaseGradientHandler(ABC):
+    """A basic helper class to handle all-reduce operations of gradients across different parallel groups
+    before optimization.
+
+    Args:
+        model (Module): Model where the gradients accumulate.
+        optimizer (Optimizer): Optimizer for updating the parameters.
+    """
+
+    def __init__(self, model, optimizer):
+        self._model = model
+        self._optimizer = optimizer
+
+    @abstractmethod
+    def handle_gradient(self):
+        """A method to accumulate gradients across different parallel groups. Users should
+        write their own functions or just use the functions in pre-defined subclasses.
+        """
+        pass