[cherru-pick] add use_dilation params in hubserving

cherry-pick fix doc and fix dilation

[cherry-pick #1920]

优化部署、安装相关文档

Update joinus.png

Add FAQ for 2.8

Update joinus.png

[cherry-pick] predict_rec support rare and fix rare export error

fix srn for eval

update srn dataset path

Fix encode for srn and attn

polish code for srn eval

[cherrp-pick]update rare dataset path

[cherry-pick] Update requirements 

[cherry-pick] Check empty data

[cherry-pick] fix divide zero error