Custom nodes may be places in `$INVOKEAI_ROOT/nodes/` (configurable with `custom_nodes_dir` option).
On app startup, an `__init__.py` is copied into the custom nodes dir, which recursively loads all python files in the directory as modules (files starting with `_` are ignored). The custom nodes dir is now a python module itself.
When we `from invocations import *` to load init all invocations, we load the custom nodes dir, registering all custom nodes.
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