quantization: deprecate observer compute_dtype and replace with is_dynamic (#85431)
Summary:
This PR deprecates the `compute_dtype` field on observers, and replaces
it with the `is_dynamic` field on observers. This is better aligned
with the reference model spec.
Test plan:
```
python test/test_quantization.py TestQuantizeFx
python test/test_quantization.py TestQuantizeFxOps
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85431
Approved by: https://github.com/jerryzh168
