FixedPoint `symmetric=True` Min Value

[awgu]

https://github.com/Tiiiger/QPyTorch/blob/176bf2a4b02021b116991d9b2f4d920861725f26/qtorch/number.py#L23

Should this be [-2^{wl-fl-1}+2^{-fl}, 2^{wl-fl-1}-2^{-fl}]? This seems to be how symmetric fixed-point is used in the WAGE paper (bottom of page 3).

For the current formula, if wl=8 and fl=1, then the min value is -2^{8-7-1}^{-7} = -2^{0}^{-7} = -2^{0} = -1, which is the same as if symmetric=False.