[Quant][docs] Fix BackendConfig example in docstring/README (#89319)
Summary: The example in the BackendConfig docstring and the README
was not runnable. This fixes a typo (`bias_type` -> `bias_dtype`),
removes the call to an internal helper function, and adds an
additional BackendPatternConfig to make the example BackendConfig
more realistic and useful.
Reviewers: jerryzh168, vkuzo
Subscribers: jerryzh168, vkuzo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89319
Approved by: https://github.com/jerryzh168
