Values X1, X2 empty in the first

[haithanhp]

Dear @kirthevasank,

Thanks for your great implementation. I try to run the code with demos/demo_cnn.py. But I see X1, X2 is empty to compute the distance, leading to a bug in computation.
In file opt/nasbot.py
init_gp_fitter = NNGPFitter([], [], self.domain.get_type(), tp_comp=self.tp_comp,
list_of_dists=None, options=self.options,
reporter=self.reporter)

Could you help to solve this issue? Thanks so much.