[Quant][docs] Move parts of BackendConfig tutorial (#91999)
Summary: This commit moves the API specification section of
the BackendConfig tutorial to the docstrings, which is a more
suitable place for this content. This change also reduces some
duplication. There is no new content added in this change.
Reviewers: jerryzh168, vkuzo
Subscribers: jerryzh168, vkuzo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91999
Approved by: https://github.com/vkuzo, https://github.com/jerryzh168
