[feat] Dynamic import based on the available dependencies

[lolipopshock]

After this update, layout-parser would support the _LazyModule(originally from the transformers repo) that enables more flexible imports and dependency management.

For all OCR and LayoutModels, it may require additional dependencies installed beyond the base layout-parser implementation. Previously we resort to the method _import_module that binds the loading of the external dependencies to the specific classes. As a result, the import of the required dependencies should be handled within the classes, adding complexity to the model implementation.

In the new updates, the additional dependencies is handled at the library level: if the dependencies is not available, the actual python file for the corresponding class won't be imported and loaded as a part of the library. For example, if Detectron2 is not installed, after running import layoutparser as lp, the lp object even won't have the attribute Detectron2LayoutModel. This decouples the requirements with the corresponding class, and makes it easier to implement new models.