Allow optimum to discover and load subpackages (#1894)
As an alternative to directly adding their commands in a register.py file under
the root optimum directory, this adds a decorator to declare a subcommand that
can be used by subpackages when they are loaded.
This will fix the issue of subcommands 'disappearing' when optimum is upgraded
without reinstalling the subpackage.
The onnxruntime commands are moved into a subpackage loader directory.
This subpackage directory is only loaded (and its commands added) when
the onnxruntime is available.
This avoids wrongly indicating that the onnxruntime commands are available
when the package is actually not installed.
