Photodiode Equivalent Circuit
This circuit is just the photodiode model and a load resistance. No bias network.

The ideal diode is there because this is a photodiode. Which means that if you forward bias it, a lot of compoments gets bypassed, but when you reverse bias it those components come into play.

The shunt resistance is really high. It's basically the resistance across the PN junction when the PN junction doesn't want to conduct, I believe. It's the resistance that everything has, even when it really doesn't want to conduct anything.