Make util function and classes public and simplify import path (#2129)

* Make util function and classes public and simplify import path

Summary:
Some of the function and classes are used for building quantizers, so users may have to import from other files like
`torchao.quantization.pt2e.graph_utils`, this PR makes functions in
`export_utils`, `graph_utils`, `observer`, `fake_quant` available in `torchao.quantization.pt2e`

and functions in `torchao.quantization.pt2e.qunatizer.utils` available in `torchao.quantization.pt2e.quantizer`
and made them public

Test Plan:
pytest test/quantization/pt2e

Reviewers:

Subscribers:

Tasks:

Tags:

* fix _xnnpack quantizer

* fix imports

* update pre commit version

* trigger pre-commit