ns for fx: move unmatchable mod/fun/meth mapping to mappings file (#56197)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56197
No logic change, just moving code around.
Test Plan:
```
python test/test_quantization.py TestFXNumericSuiteCoreAPIs.test_op_io_dtype_coverage
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D27805332
fbshipit-source-id: 0a63cf6ef7e5c4f655cdd5a18d54cc988424ac80
