append and prepend not supported yet

remove the prepend and append checking interface from the backend

refine the code, enrich the test set and all tests passed

registered the diff operator into npi scope

all tests passed

comments and minor modification

format codes and fix warning for sanity check

minor modification for sanity check

fix sanity

fix the tolerance bound of testing np.diff

resolve minor coding style issue

replace the given tests by random picking

minor fix