remove legacy from docs (#15112)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/15062
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15112
Differential Revision: D13547845
Pulled By: soumith
fbshipit-source-id: 61e3e6c6b0f6b6b3d571bee02db2938ea9698c99
