quantization: align observer dtype with reference model spec (#85345)
Summary:
Before this PR, the `dtype` attribute of observers was not clearly
defined. It originally meant `interface_dtype` in the eager mode
workflow, which is how the codebase before this PR is using it.
In the new reference model spec, `dtype` attribute of an observer
represents the `dtype` value which needs to be passed into a `quantize`
function in the reference model spec. This PR aligns the codebase
to this definition of dtype. In detail:
1. change util functions to interpret `dtype` using the reference model definition
2. change `prepare` to interpret `dtype` using the reference model definition
3. change observers for dynamic quantization to interpret `dtype` using the reference
model definition.
A future PR (left out of this one to keep LOC small) will deprecate the
`compute_dtype` field and instead expose `is_dynamic` on observers.
"
Test plan:
```
python test/test_quantization.py TestQuantizeFx
python test/test_quantization.py TestQuantizeFxOps
```
Differential Revision: [D39675209](https://our.internmc.facebook.com/intern/diff/D39675209)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85345
Approved by: https://github.com/z-a-f, https://github.com/jerryzh168
