feat: add logit-normal schedule

[stduhpf]

Summary

Adds support for the logit-normal schedule that supposed to be the default noise schedule for Ideogram4 (see https://github.com/ideogram-oss/ideogram4/blob/main/docs/pipeline.md#noise-schedule).

It's now set as default scheduler when running an Ideogram4 model.

Related Issue / Discussion

N/A

Additional Information

Uses Acklam's Algorithm to approximate the Inverse Normal CDF to avoid adding a boost dependency.

Discrete scheduler	Logit-normal (mu=0,std=1.75)

I'm noticing the results seem pretty consistently slightly worse with (my implementaion of) this schedule, so I don't know if I'm doing something wrong, or if it's just not that good despite being recommended in the official implementation.

Checklist

• I have read and confirmed this PR follows the contribution guidelines.

[stduhpf]

I played around with the formula a bit, by carefully chosing the mean and std parameters, it seems to be possible to approximate roughly any other schedule...

Examples:

mean	std	closest schedule
0	1.7	discrete
0	2.6	smoothstep
3.3	2.6	exponential

https://www.desmos.com/calculator/irnfqihjqa

For some reason, the recommended parameters seem to give subpar results compared to discrete (the current default), especially at high resolution (at 512x512 is should be pretty close but the resolution-aware thing steers it away at higher resolution).

Maybe it's just that I'm comparing with prompts/seeds that were cherry-picked because they were working great with the discrete scheduler.