[quant][graphmode][fx] Fix getitem for unmatched nodes (#57173)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57173
If getitem is followed by an unmatched node, we'll remove the observer after it.
Test Plan:
python test/test_quantization.pyt TestQuantizeFxOps.test_getitem
Imported from OSS
Reviewed By: vkuzo
Differential Revision: D28068805
fbshipit-source-id: e79f8ec3e8fd61d348b8a7069ab0bb434d737c30
