Implement Observer pass on simple model and validate stats (#18848)

Implement Observer pass on simple model and validate stats (#18848)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18848

The Observer Module is based on eager mode compute qparam implementation.
Goal is to validate QParam result for EagerMode and Script Mode for simple
model

Observer stats are collected and qparam computed only for activations only at this point

Reviewed By: zafartahirov

Differential Revision: D14720805

fbshipit-source-id: cb2f321b4b9927b37905fdb8eb55c5610d41b351