'high' and/or 'low' VRAM usage modes

[tazlin]

This is more pertinent to the worker use case but certain recent changes by ComfyUI and myself have highlighted that there could be some better finessing of memory conditions. While I do feel that it should be largely in the realm of ComfyUI internals to manage this, that project has changed or added a large amount of functionality which is harmful to the worker use case.

• In particular the change in commit 4e99a01 highlights that there should be at least a 'high vram' mode which opts-in to either default comfy behavior or the deliberately more aggressive use of memory we occasionally use.