* update dygraph api-doc and backward api-doc, test=develop

* update dygraph api-doc and backward api-doc, update api.spec, test=develop

* update dygraph api-doc and backward api-doc, update api.spec, test=develop

* update API.spec, test=develop

* fix dygraph mem leak, test=develop

* polish msg, test=develop

* add api commet; test=develop

* fix fc dtype bug; test=develop

* remove float32 in default parameter; test=develop

* fix exmpale bug; test=develop

* fix build once; test=develop

* fix num_chanels bug; test=develop

* fix install check failed bug; test=develop

* save optimizer related vars in dygraph

* test=develop, add optimizer save and load

* test=develop, add optimizer save and load

* test=develop, merge code and add multi-optimizer save and load

* test=develop, fix test_imperative_checkpoint

* test=develop, fix include error

* test=develop, fix include error

* test=develop, renew api spec

* test=develop, refine code

* test=develop, set default value for checkpoint

* test=develop, fix ci error

* test=develop, change API.spec and make api more readable

* test=develop, refine version and time stamp

* test=develop, add example code and refine code

* test=develop, refine doc

* test=develop, change version

test=develop

test=develop

test=develop

test=develop

test=develop