[LoRA] Enabling limited LoRA support for text encoder (#2918)
* add: first draft for a better LoRA enabler.
* make fix-copies.
* feat: backward compatibility.
* add: entry to the docs.
* add: tests.
* fix: docs.
* fix: norm group test for UNet3D.
* feat: add support for flat dicts.
* add depcrcation message instead of warning.
