[quant][[fx] update observer_fqn to not depend on node.name (#66767)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66767
Make observer fqn in prepare step independent of input_node/observed_node name.
This change names the observers as `{input/output}_activation_post_process_{idx}` where idx will be incremented for each new observer instance and is guaranteed to be unique.
Test Plan:
python test/test_quantization.py test_observer_fqn
Imported from OSS
Reviewed By: anjali411
Differential Revision: D31752052
fbshipit-source-id: e0995b1ef33a99d5b012133fe92d303d55a73b7d
