Open drain input in I2C HDL module?
The code in the module gives you a pretty good idea as to what this does, it selects between whether the output is being pulled high or if its sending data. 

With the open_drain signal you can pull the outputs high external to the module. 

input open_drain, // Open drain

assign scl_out = open_drain ? 1'b0 : scl_count[1];


Another note is on some FPGA's you will need an additional module that changes the pin into a high impedance state required by I2C.