[quant][pt2] Preserve source_fn_stack after QAT fusion (#110899)
Test Plan:
python test/test_quantization.py TestQuantizePT2EQAT.test_qat_preserve_source_fn_stack
Reviewers: jerryzh168, kimishpatel
Subscribers: jerryzh168, kimishpatel, supriyar
Differential Revision: [D50101253](https://our.internmc.facebook.com/intern/diff/D50101253)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110899
Approved by: https://github.com/jerryzh168
