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: @yanzhenxiang2020
Reviewed-by: 
Signed-off-by:

From: @liu_xiao_93
Reviewed-by: @liangchenghui
Signed-off-by: @liangchenghui

From: @jiangzg001
Reviewed-by: @liangchenghui,@wuxuejian
Signed-off-by: @liangchenghui

From: @jiangzg001
Reviewed-by: @liangchenghui
Signed-off-by: @liangchenghui

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

From: @lianliguang
Reviewed-by: @zh_qh,@zh_qh,@chujinjin
Signed-off-by: @zh_qh,@zh_qh

From: @jiangzg001
Reviewed-by: @liangchenghui,@wuxuejian
Signed-off-by: @liangchenghui

A minor change in sigmoidGradGrad operation (non-functional change).

From: @liangchenghui
Reviewed-by: @c_34,@wuxuejian
Signed-off-by: @wuxuejian

check input dims for nn.LSTM.

From: @david-he91
Reviewed-by: @liangchenghui,@linqingke
Signed-off-by: @liangchenghui

From: @david-he91
Reviewed-by: 
Signed-off-by:

From: @jiangzg001
Reviewed-by: 
Signed-off-by: