[Quant][docs] Use get_default_qconfig_mapping (#87299)
Summary: The recommended way to use QConfigMapping is through
`get_default_qconfig_mapping`. However, the docs still references
usages that use `QConfigMapping().set_global(...)`. This doesn't
actually work well in practice when the model has fixed qparams
ops for example. This commit updates these usages.
Reviewers: vkuzo
Subscribers: vkuzo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87299
Approved by: https://github.com/jerryzh168
