Document limitations & common model mutation approaches for export compliance

[GregoryComer]

📚 The doc issue

Model exportability is a currently a pretty tricky topic. While this is largely owned by PyTorch core, it affects us heavily as export consumers, and we don't have the luxury of eager mode fallback. Let's add a page to the documentation to cover exportability at a high level and talk about common patterns that cause problems (esp. data dependent control flow + control flow unrolling in general). We can provide workarounds / common solutions. This will hopeful reduce the exportability pain a little bit by giving user knowledge of what isn't really supported and how they can move forward.

Suggest a potential alternative/fix

No response

cc @mergennachin @byjlw