What does the VarLengthMultiheadSA do to SparseTrTensor input

Hi, the project is very helpful. thank you. 
I'd like to know what the operations are in the _VarLengthMultiheadSA_ and how to utilize this module to build a network. 
Could you provide an example? the two projects you mentioned do not use this function to create a transformer-based network. 
Thank you again.