Delete dead TensorImpl::detach_autograd_meta (#28609)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28609
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Differential Revision: D18171159
Pulled By: ezyang
fbshipit-source-id: 509061ca56186c7762da9634abecbafad0277d94
