code: public Observable<PersistedFace> getFaceAsync(String personGroupId, UUID personId, UUID persistedFaceId) {
          return getFaceWithServiceResponseAsync(personGroupId, personId, persistedFaceId).map(new Func1<ServiceResponse<PersistedFace>, PersistedFace>() {
              @Override
              public PersistedFace call(ServiceResponse<PersistedFace> response) {
                  return response.body();
              }
          });
      }
docs: Retrieve information about a persisted face (specified by persistedFaceId, personId and its belonging personGroupId).
  
  @param personGroupId Id referencing a particular person group.
  @param personId Id referencing a particular person.
  @param persistedFaceId Id referencing a particular persistedFaceId of an existing face.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the PersistedFace object