Investigate compile time issue with siggraph neural shading sample

[jarcherNV]

Ths siggraph neural shading samples have a fairly significant compile time problem with one of the shaders.
The samples can be found at: https://github.com/shader-slang/neural-shading-s25

The specific sample is the Slangpy network/nsc_nw_02_3layers.py example
This sample appears to "hang" on startup, but it is actually waiting for the shaders to compile.

The shaders themselves are not particularly large, so we should determine why they are taking so long to compile and fix it if possible.

So far I have repro'd this on Windows, and Kai has repro'd on Linux.

[tunabrain]

The issue was mitigated in the sample code for the SIGGRAPH course by using [MaxIters(n)] instead of [ForceUnroll].

The problem is caused by DXC taking a long time to compile even small neural networks when loop unrolling is also used. We can fix it for the sample by not using loop unrolling.

The remaining action item is to document this in the sample code so people are aware of this issue when they build their own networks.