[PT2E][Quant] Refactor quantizer and qnnpack qantizer code to support dqlinear config

Pull Request resolved: #99399

This diff introduces a few refactors:

- Move observer creation to utils.py.
- Use quantization spec to supply args to observers.
- Use annotation function registration corresponding QuantizationConfig. This
  will be later used in dynamic quantized linear.
ghstack-source-id: 187514867

Differential Revision: [D45073790](https://our.internmc.facebook.com/intern/diff/D45073790/)

Author: kimishpatel