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

Update program.py

update docker

PaddleOCR/tools/program.py的第254行的变量名原为cur_metirc，需要修改为cur_metric，如果不修改在运行是，会提示
The variable name of line 254 can be changed to cur_ Metric. If you do not modify it, you will be prompted when it is running
"E:\XXX\PaddleOCR\tools\program.py", line 257, in train
['{}: {}'.format(k, v) for k, v in cur_metric.items()]))
NameError: name 'cur_metric' is not defined