* add sampler in examples and add user guide file.

* Add neighbor_samplers, including NodeNeighborLoader and LinkNeighborLoader, supporting Graph and HeteroGraph  neighbor sample.

* add GraphSAINTSampler and random_walk ops, and fix cpp extensions layout.

* third_party: add parallel_hashmap.

* fix README doc and some version details.

* fix README doc and some version details.

* fix README doc and some version details.

* Update docs

* Fix sampling bugs.And make some tests to make sure that framework will work without cython.Now all cython code has been removed.All datasets can be used without param 'root'. If 'root' is None, ggl will help manage dataset storage.

* Fix sampling bugs.And make some tests to make sure that framework will work without cython.Now all cython code has been removed.All datasets can be used without param 'root'. If 'root' is None, ggl will help manage dataset storage.

* Fix sampling bugs.And make some tests to make sure that framework will work without cython.Now all cython code has been removed.All datasets can be used without param 'root'. If 'root' is None, ggl will help manage dataset storage.

* fix setup.py

* add cuda extension experimental.

* add cuda extension experimental.

* fix Graph __iter__ problem.

* fix Graph __iter__ problem.

* fix CPP ops structure.

* fix type bug.

* fix build extension.

* add lazy load for paddle.

* fix extensions.

* fix setup.py

* strengthen gammagl ops compatibility.

* fix segment ops.

* fix windows compile bugs.

* add metapath2vec and herec model

* fix bugs

* rename

---------

Co-authored-by: dddg617 <996179900@qq.com>
Co-authored-by: BuptTab <gyzhou2000@gmail.com>
Co-authored-by: dddg617 <75086617+dddg617@users.noreply.github.com>