revise wor2dvec, will move fast training to a separate chapter
revise wor2dvec, will move fast training to a separate chapter

mli/gluon-tutorials-zh,d2l-ai/d2l-zh,mli/gluon-tutorials-zh,mli/gluon-tutorials-zh,d2l-ai/d2l-zh,mli/gluon-tutorials-zh,d2l-ai/d2l-zh