[caffe2] Fix the correctness check for GivenTensorFill operator
Summary:
DCHECK is never triggered and the user error could lead to crash.
I could make the error message be even nicer by checking shape in contructor, but even this would do.
Reviewed By: m3rlin45
Differential Revision: D21778992
fbshipit-source-id: a8ec2faaf734746f6dc42879705245851dc99bed
