polish (#392)
polish (#392)

* fix function f(x)=x_1^2+2x_2^2 in chapter_optimization gd-sgd.md

* explain the meaning of x in chapter optimization gd-sgd.md

* minor polishing

* minor polishing

* Revert "minor polishing (#383)"

This reverts commit 3956f1fda1a720743b7dbc15e5b4ffd2b11cf076.

* polishing

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