Created by: sneaxiy

Add Eigen::internal::index_op<T> support to Eigen.

Suppose x and y are TensorMaps of Eigen, y.device(device) = x.nullaryExpr(Eigen::internal::index_op<int>()) would produce the sequence 0,1,2,...,numel(x)-1.

This Eigen::internal::index_op<T> could be used to replace broadcast() operation in Eigen, making some op faster.