docs: change to FloatTensor for `requires_grad=True` (#54658)
Summary:
fixes https://github.com/pytorch/pytorch/issues/54506
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54658
Reviewed By: ailzhang
Differential Revision: D27328321
Pulled By: zou3519
fbshipit-source-id: d29fa266a1cb2b6d8566055dfb6ce001edde9d96
