chore: Update ModelReportObserver variables to buffers (#97971)
This commit changes ModelReportObserver variables to buffers similar to other observers. This will allow for gathering data on other device than CPU.
Moreover, updates InputWeightEqualizationDetector to compute weight stats that are on GPU
Tested with running tests `test/quantization/fx/test_model_report_fx.py`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97971
Approved by: https://github.com/vkuzo
