What I would do is to enable our TA to be able to comment students' assignments, but not grade them.
At admin permission page, for role TA, I disable "Edit grade", and keep "View all students' submissions and make comments on them" enabled.
Then I tried an assignment, the Assessment (Grade __ out of xxx) box disappeared, as expected. But when I tried to submit a comments, it didn't work and said "Oops! The last request doesn't work out." (see below the screenshot).
I've been trying all the ways but can't sort it out. Could somebody here offer some help? Thank you!