当x，y和theta都是向量的时候如何计算损失

function J = computeCost(X, y, theta)
%COMPUTECOST Compute cost for linear regression
%   J = COMPUTECOST(X, y, theta) computes the cost of using theta as the
%   parameter for linear regression to fit the data points in X and y

% Initialize some useful values
m = length(y); % number of training examples

% You need to return the following variables correctly
J = (X*theta-y)'*(X*theta-y)/(2*m);

% ====================== YOUR CODE HERE ======================
% Instructions: Compute the cost of a particular choice of theta
%               You should set J to the cost.

% =========================================================================

end

不能使用（X*theta）^2也不需要使用sum，注意矩阵的shape要对上不然计算时会出错