[AO] Cubic sparsity level scheduler (#85232)
The scheduler updates the levels of sparsity based on https://arxiv.org/abs/1710.01878.
## Implementation
The update rule is defined as:
$$
\begin{aligned}
s_t &= s_f + (s_i - s_f)\left( 1 - \frac{t - t_0}{n\Delta t} \right)^3 \\
\mbox{for} ~ t &\in \left\\{ t_0, t_0+\Delta t, \dots, t_0 + n\Delta t \right\\} \end{aligned}
$$
There is a minor difference compared to the original paper. By providing `initially_zero` argument, one can set the level of sparsity before step $t_0$: If `False`, the sparsity level before $t_0$ is set to $s_i$, otherwise 0.
## Tests
```
python test/test_ao_sparsity.py -- TestCubicScheduler
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85232
Approved by: https://github.com/junesg, https://github.com/jerryzh168
