This question was recently brought up in another post:
There is a WIP pull request to implement this: https://github.com/pymc-devs/pymc3/pull/2983 But we met some complication as there are bugs with the repeated shape/size.
For now, you can use this old (slow) implementation
https://github.com/junpenglao/Planet_Sakaar_Data_Science/blob/master/WIP/Test_sample_prior.ipynb
I am not sure if any progress has been made recently (cc @colcarroll). Maybe it is worth to implemented the slower version as reference for now?