vpr/buf/buf.pb_type.xml

<pb_type name="BUF" num_pb="1">
  <input  name="I" num_pins="1"/>
  <output name="O" num_pins="1"/>
  <interconnect>
   <direct name="IO" input="BUF.I" output="BUF.O"/>
 </interconnect>
</pb_type>