[Modular] update the collection behavior

[yiyixuxu]

We guarantee unique component names under each collection, i.e. for each collection, there is only one "unet", one "vae" ....

Currently, when a new component enters the collection, we remove the existing one
e.g. node1 has a "unet" that's sdxl default, when we add a new unet from node1, say "CyberRealisticXL", the sdxl default unet will be removed from the components manager

This is wrong, and this PR changes the behavior so that we simply dissociate the existing from the collection, and won't delete it if the component is still associated with a different collection

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[asomoza]

LGTM, thanks!