Rename 'torch/ao/nn/quantized._reference' to 'torch/ao/nn/quantized/reference'. (#84974)
Currently, the path for reference modules contains _ which means it's private (https://github.com/pytorch/pytorch/tree/master/torch/ao/nn/quantized/_reference), but we would like to make it public since the reference module is now enabled by default in the fx graph mode quantization flow and it will be added to eager mode flow as well in the future.
To make '_reference' public, it should satisfy the [public API rules](https://github.com/pytorch/pytorch/wiki/Public-API-definition-and-documentation).
I did in the first commit (prepare '_reference' to be public):
1: add __all__ to public modules and packages;
2. made functions, that are only used in the file that the function is defined, private by adding _ at their names.
Fixes #83090. (we rename the 'torch/ao/nn/quantized/_reference', because of migration #81667.)
This is a dup for the #84786.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84974
Approved by: https://github.com/andrewor14, https://github.com/z-a-f
