add dataset len check

fix typo for encode

fix divide zero error

improve doc

Update joinus.png

fix typo for attention

Update joinus.png

fix doc

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

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