Common model for runners

[Ron Gonzalez]

Hi,  When I build a data flow using the Beam SDK, can someone point me to the 
code that represents the underlying representation of the beam model itself?  
Is there an API that lets me retrieve the underlying protobuf-based graph for 
the data flow? Perhaps some pointers to what code in the runner retrieves this 
model in order to execute it in the specific engine?
Thanks,Ron