Gradient Checking
 Backprop sometimes has bugs. Gradient checking will check wheter backprop will eventually decreasing.....
 this will teach how 100% accurate backprop without bug....

Takes an error based on epsilon  Two sided difference will make better correction than one sided difference
 3+10^4
 let theta be a matrix
 the epsilon will only be used to particular theta
 this way, the particular theta that being calculated will always be corrected
 dvec is a partial derrivative from backpop that used as a cost function
 if the gradApprox is more or less same as Dvec, then the derrivative will surely output the right result
 Here's the step on how to implement numerical gradient checking
 The important things is to be sure to shut gradient checking after checking the backprop. This is intended because gradient checking is computationally expensive
 Backprop code is much more efficient and faster
 These method (gradient checking) is to makes sure the code that we made, is correctly decreasing from backprop value