According to https://eigen.tuxfamily.org/dox/TopicPitfalls.html one
should just avoid using `auto` as the type of an Eigen expression.
This PR fixes most of them I could found in the project. There might be
cases that I missed, and I might update those later if I noticed.
This should prevent issues like #7741 and hopefully fix some mysterious
crashes happened inside Eigen calls.
