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: @chujinjin
Reviewed-by: @jjfeing,@kisnwang
Signed-off-by: @kisnwang

From: @liubuyu
Reviewed-by: @jjfeing
Signed-off-by:

Signed-off-by: zhoufeng <zhoufeng54@huawei.com>

Signed-off-by: zhoufeng <zhoufeng54@huawei.com>

Merge pull request !2989 from kisnwang/two-step-reduce-sparse-gradient