[Static Runtime] documentation update for StaticRuntime (#81066)
Summary:
- Documentation for Asynchronous execution in StaticRuntime Engine
- Added summary for ProcessedNodeMetadata structure, StaticRuntime::runAsync() API, prim::fork and aten::wait operations
Differential Revision: D37700998
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81066
Approved by: https://github.com/tenpercent, https://github.com/mikeiovine
