Handling nan output from Adam optimizer when generating reference parameters

[xuyuon]

When trying to generate a set of reference parameters for the heterodyning likelihood, the output list of log likelihood often contains some nan values. When we try to search for the minimum log likelihood using argmin() on a list containing nan values, it will result in a list of nan as the reference parameters, causing the program to run into a bug. I think it would be a better idea to replace argmin() with nanargmin(). This will allow the code to search for the minimum log likelihood while ignoring the nan values. This approach will enable the code to proceed, but it should still output a warning to notify the user that nan values were present in the log likelihood list.