[quant][be] Add typing for quantization_mappings.py (#49179)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/49179
Test Plan: Imported from OSS
Reviewed By: vkuzo, wat3rBro
Differential Revision: D25470520
fbshipit-source-id: 16e35fec9a5f3339860bd2305ae8ffdd8e2dfaf7
