[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/_zero_gradient_handler.py

@@ -0,0 +1,21 @@
+from colossalai.registry import GRADIENT_HANDLER
+
+from ._base_gradient_handler import BaseGradientHandler
+
+
+@GRADIENT_HANDLER.register_module
+class ZeROGradientHandler(BaseGradientHandler):
+    """A helper class to handle all-reduce operations in a data parallel group.
+    A all-reduce collective communication will be operated in
+    :func:`handle_gradient` among a data parallel group.
+    This class is specialized with ZeRO optimization.
+
+    Args:
+        model (Module): Model where the gradients accumulate.
+        optimizer (Optimizer): Optimizer for updating the parameters.
+    """
+
+    def handle_gradient(self):
+        """A method running a all-reduce operation in a data parallel group.
+        """
+        self._optimizer.sync_grad()