Update link address deep-learning-intro.md

Update link address of Momenta company.

Update use-gpu.md

fix buy -gpu.md some errors

remove the last section. too abrupt. will make smoother introduction in the book contents.

更正math.md标量与矩阵乘法中，各元素的下标

log_rmse()是一个泛用的损失函数，其定义不应该只接受train_features，而且事实上在后面`def train()`函数里也调用了`log_rmse(net, test_features, test_labels)`