Fix circular imports related to SCHEDULER_NAME_VALUES

[RyanJDick]

Summary

This PR makes the following changes:

• Moves SCHEDULER_NAME_VALUES to invokeai/backend/. Previously, it was located in invokeai/app/invocations/. In its old location, it was causing bidirectional imports between invokeai/backend/ and invokeai/app/invocations/ and thus contributing to circular import issues. This change helps to move towards having invokeai/app/invocations/ import from invokeai/backend/ and not the other way around. (More info on why this is important here: Improve InvokeAI import patterns #6575)
• Fixes all the static type checking errors caused by doing Literal[tuple(SCHEDULER_MAP.keys())].

Related Issues / Discussions

An issue was raised on Discord where custom nodes fail to import due to a circular import error: https://discord.com/channels/1020123559063990373/1049495067846524939/1258727169417543752

This custom node import issue was introduced by #6576 , which began enforcing absolute imports. The change to absolute imports changed the order of imports in some files, which caused the circular import - highlighting yet again the fragility of the current import system.

Note: This change fixes one source of circular imports. I'm sure that others exist. I'm hopeful that this will fix most custom nodes, but there's a risk that it won't.

QA Instructions

I tested this change with the https://github.com/skunkworxdark/XYGrid_nodes custom nodes. On main, loading these nodes caused an import error. After this PR, I was able to run this workflow with custom nodes successfully: https://github.com/skunkworxdark/XYGrid_nodes/blob/ac72baf3472f47112fbbcb9ef0eff574a0ee267b/workflows/xygrid_model-cfg_wf.json

I also verified that the list of Schedulers still populates properly in the UI (both on nodes, and in the Linear UI).

Merge Plan

No special instructions.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)