From: @zhangbuxue
Reviewed-by: 
Signed-off-by:

From: @zhang_yi2020
Reviewed-by: @gemini524,@liangchenghui,@wuxuejian
Signed-off-by: @liangchenghui,@wuxuejian

1. set class GraphKernel as deprecated, and treat it as Cell
2. set class InplaceAssign as deprecated, suggested using Assign instead.
3. set op_selector as deprecated, removed the _selected_ops and _selected_grad_ops, replaced with real operations
4. removed the two passes of GraphKernel from frontend
5. removed the GraphKernel's codes from other modules

From: @linqingke
Reviewed-by: @guoqi1024,@xu-yfei
Signed-off-by: @xu-yfei

add gc.

remove gc in less_bn

[Less BN]Add GC optimizer.

update format.

fix bug.

From: @lijiaqi0612
Reviewed-by: @kisnwang,@kingxian
Signed-off-by: @kingxian

From: @gong_zi_yan
Reviewed-by: @kisnwang,@stsuteng
Signed-off-by: @stsuteng

From: @caozhou_huawei
Reviewed-by: 
Signed-off-by:

From: @lijiaqi0612
Reviewed-by: 
Signed-off-by:

The basic idea is: exploits data dependency to control the execution order
of side-effect operations, and keep the semantics of ANF unchanged.

The ControlDepend primitive is removed and there are two primitives added:

1. UpdateState:
```
  a = Assign(para, value)
```
became:
```
  a = Assign(para, value, u)
  u = UpdateState(u, a)
```

2. Load:
```
  x = Add(para, value)
```
became:
```
  p = Load(para, u)
  x = Add(p, value)
  u = UpdateState(u, p)
```

From: @fangzehua
Reviewed-by: 
Signed-off-by:

From: @xuguoyang5566
Reviewed-by: @wuxuejian
Signed-off-by:

From: @yanglf1121
Reviewed-by: @kingxian,@zhunaipan
Signed-off-by: @kingxian