[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

[cherry-pick]fix typo for encode

fix typo for attention

fix doc

delete slim related content

fix link

2021.2.1 update Faq

upload rare code