Calculate Autocorrelation Function

Calculate the autocorrelation function of the input vector using Matlab built-in function circshift, so it is very fast.
autocorr(x): compute the ordinary autocorrelation function.
autocorr_circular(x): compute the circular autocorrelation function
autocorr_fft(x): compute the circulat autocorrelation function using FFT, the result should be same as autocorr_circular.