Get KL-divergence for CSIS


I am new to Pyro and wondering how we can get the exact value from the objective function (expected forward KL divergence) when using Compiled Sequential Importance Sampling (CSIS). If I am understanding correctly, the objective function of CSIS is

(from the paper Inference Compilation and Universal Probabilistic Programming) and the loss term from Pyro CSIS implementation is evaluating the first term, which excludes the constant term. I am wondering how can we evaluate the constant term and get the exact value of the objective function, which is the expected forward KL-divergence?

I think the constant is the value of
How can we evaluate it?

Thank you in advance!