[Here](http://web.mit.edu/6.012/www/SP07-L19.pdf) they have defined as the internal resistance of current source.

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/Vig9j.png

As in the circuit g<sub>mb</sub> and r<sub>0</sub> is already included to capture the effect of parasitic and body-bias. So this can have a chance. But still, I am not sure whether these two documents are following the same notation.