This reverts commit 141c39b71c.

Change backend execution order sorting policy from DFS to BFS
in KernelGraph::SetExecOrderByDefault().

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: @liangzhibo
Reviewed-by: @ginfung,@zh_qh
Signed-off-by: @zh_qh

From: @Somnus2020
Reviewed-by: @kingxian
Signed-off-by: @kingxian

Merge pull request !6233 from yoonlee666/master

Merge pull request !5665 from yoonlee666/bugfix

Merge pull request !4043 from hexia/serving_st

1. add case to guard precision
2. fix a shape bug
3. fix a funcGraph bug

Merge pull request !1790 from lianliguang/remove-the-useless-transdata-connected-with-the-control-depend

Merge pull request !1933 from yoonlee666/edit-example