Update trainer.py

                s_logvar, d_post_mean, d_post_logvar, d_prior_mean, d_prior_logvar):

        batch_size = original_seq.size(0)

        # See https://arxiv.org/pdf/1606.05908.pdf, Page 9, Section 2.2 for details.

        # The constant items in the loss function (not the coefficients) can be any number here, or even omitted        

        # due to they have no any impact on gradientis propagation during training. So do in testing.

        # log(N(x|mu,var))

        # = log{1/(sqrt(2*pi)*var)exp{-(x-mu)^2/(2*var^2)}} 

        # = -0.5*{log(2*pi)+2*log(var)+[(x-mu)/exp{log(var)}]^2}