build: improve packaging

[qued]

PR to make to make packaging a little nicer, in line with how packaging works in unstructured. In particular, support for extras is added.

This consists of:

• Adding the file sg.in for dependencies related to supergradients. This file has been pip-compiled with output sg.txt.
• All dependencies have been pip-compiled.
• Adding the file constraints.in to add a numpy constraint that is required for compliance with supergradients. Dependencies that precede sg.in need to know about this to avoid conflicts. constraints.in is added as a constraints file for all other dependency files.
• make targets are added for installing supergradients dependencies.
• CI is updated to install supergradients dependencies.
• supergradients extra is added to setup.py and MANIFEST.in so it can be picked up by PyPi.
• To support the fact that the supergradients dependencies are not guaranteed to be installed, the way imports are handled in the super_gradients.py model file has been changed so imports are only attempted in the model constructor, in a module-level global variable. The constructor was used instead of doing the import in initialize due to an error that occurred during import when initiated from initialize.

Testing

From a fresh pyenv environment, and from the base folder of unstructured-inference:

pip install -e .
pip list | grep gradient

(There should be no output, as supergradients should not be installed.)

Now:

pip install -e ."[supergradients]"
pip list | grep gradient

(Output should show super-gradients is installed.)

[pravin-unstructured]

Looks good to me.