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: @huangbingjian
Reviewed-by: 
Signed-off-by:

From: @linqingke
Reviewed-by: @zh_qh,@guoqi1024
Signed-off-by: @guoqi1024

From: @huangbingjian
Reviewed-by: @ginfung,@zh_qh
Signed-off-by: @zh_qh

From: @Margaret_wangrui
Reviewed-by: @zh_qh,@hwhewei
Signed-off-by: @zh_qh

Replace US with its monad input in DeleteLoadUserUpdateState().

From: @xychow
Reviewed-by: @zh_qh,@ginfung
Signed-off-by: @zh_qh

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

From: @chenfei52
Reviewed-by: @zh_qh
Signed-off-by: @zh_qh

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

From: @chenfei52
Reviewed-by: 
Signed-off-by:

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

From: @chenfei52
Reviewed-by: @ginfung,@zh_qh
Signed-off-by: @zh_qh

From: @zh_qh
Reviewed-by: @ginfung,@hwhewei
Signed-off-by: @ginfung

From: @hwhewei
Reviewed-by: @ginfung
Signed-off-by:

All Load nodes in a MakeTuple should use same monad if we are going to merge UpdateStates for them.

Use more faster but not strict monad state check.