ensure that there is only one "prototype" folder in torchao

[vkuzo]

context

Today torchao has various prototype folders:

1. https://github.com/pytorch/ao/tree/main/torchao/prototype
2. https://github.com/pytorch/ao/tree/main/torchao/quantization/prototype
3. https://github.com/pytorch/ao/tree/main/torchao/sparsity/prototype

the task

This is confusing. Let's just converge on one place, torchao/prototype. This task is to move the code over without breaking things:
A. chat with @andrewor14 and confirm if it's ok to move torchao/quantization/prototype to torchao/prototype/quantization. If yes, move it.
B. chat with @jcaip and confirm if it's ok to move torchao/sparsity/prototype to torchao/prototype/sparsity. If yes, move it.

When moving the code, it would be good to verify that scripts/features/tests still work, CI is green, and update any hardcoded locations as needed.

Note: feel free to ignore torchao/experimental for now.

[vkuzo]

cc @drisspg