[quant] Removing hardcoded "torch.quantization.observer" for migration (#64981)

[quant] Removing hardcoded "torch.quantization.observer" for migration (#64981)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64981

this would have cause errors when observer.py was moved to ao.

see: D30391189
ghstack-source-id: 138118430

Test Plan:
buck test mode/opt //caffe2/test:quantization -- --exact 'caffe2/test:quantization - test_dynamic_quant_multi_uses (quantization.jit.test_quantize_jit.TestQuantizeDynamicJitPasses)'

buck test mode/opt //caffe2/test:quantization -- --exact 'caffe2/test:quantization - test_save_load_state_dict_script (quantization.core.test_workflow_module.TestObserver)'

Reviewed By: supriyar

Differential Revision: D30432008

fbshipit-source-id: 754727a89c78f6ceada6f8ff92c304f3953f38fc