How to install caffe2

Then follow the instructions here:
https://github.com/facebookresearch/Detectron/blob/master/INSTALL.md

How to fix the error:
/usr/local/include/eigen3/Eigen/Core:42:34: fatal error: math_functions.hpp: No such file or directory
compilation terminated.
CMake Error at caffe2_gpu_generated_arg_ops.cu.o.cmake:203 (message):
  Error generating

  /home/liangzou/gitcaffe2/caffe2/build/caffe2/CMakeFiles/caffe2_gpu.dir/operators/./caffe2_gpu_generated_arg_ops.cu.o

This is caused by a bug in Eigen. Use this version of Eigen to fix it:
//mirror.bazel.build/bitbucket.org/eigen/eigen/get/034b6c3e1017.tar.gz