* Add fluid benchmark document

Compare some models' performance as reference for users.
Signed-off-by: Nzhaoyuchen <zhaoyuchen01@baidu.com>

* Refine doc.
Signed-off-by: Nzhaoyuchen <zhaoyuchen01@baidu.com>

* Refine document
Signed-off-by: Nzhaoyuchen <zhaoyuchen01@baidu.com>

* Refine doc
Signed-off-by: Nzhaoyuchen <zhaoyuchen01@baidu.com>