Introduce backend module (autogl.backend)

update version in document

update readme and remove redundant file

add new features:
* NAS
* sampling
* Link prediction

Update version (functional)

Revise example and documents for sampling

Revise model tutorial and add some default hyper parameters.

Fix bugs in nas, revise example.

The words *mendatory* and *mandatory* have different meanings, take a look at dictionary for more details. I replaced the *mendatory* (possibly a typo for word *mandatory*) with *required*. The pair `optional <-> required` is more commonly used in programming.