ns for fx: move node I/O dtype mapping to be local instead of global (#57021)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57021
To support shadows of custom functions, we need to allow user to
specify I/O type of the custom functions.
This PR is a cleanup in preparation for making the above happen.
We make the I/O dtype mappings be generated by a function instead
of a global variable. In the next PR, we will add a hook so user
can modify these mappings.
Test Plan:
```
python test/test_quantization.py TestFXNumericSuiteCoreAPIs
```
Reviewed By: jerryzh168
Differential Revision: D28030094
Pulled By: vkuzo
fbshipit-source-id: 3cbb617f034ef385c2875c4ec7fed13ca30bfc57
