remove unnecessary Dense from mlp-gluon net (#271)
remove unnecessary Dense from mlp-gluon net (#271)

Change-Id: I4f410d7fb6e58201628e646b7a167ba4af11c92f
d2l-ai/d2l-zh,mli/gluon-tutorials-zh,mli/gluon-tutorials-zh,d2l-ai/d2l-zh,d2l-ai/d2l-zh,mli/gluon-tutorials-zh,mli/gluon-tutorials-zh