[quant] Modify APoT qparam quantization levels calculation (#80303)
### Summary:
This PR updates an error in the the computation for APoT quantization levels to match the formula defined in the APoT paper: https://arxiv.org/pdf/1909.13144.pdf.
### Test Plan:
Run unit tests with:` python pytorch/test/quantization/core/experimental/test_nonuniform_observer.py`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80303
Approved by: https://github.com/dzdang
