[Refactor/Docs] Organize Class docstring & Add custom exceptions

[kozistr]

Problem (Why?)

there's no proper exception class (e.g. no sparse gradient, zero parameter size)

Solution (What/How?)

• register custom exceptions
• refactor the docstrings
• support gradient centralization for Adai optimizer
• support AdamD debias for AdaPNM optimizer
• fix SAM optimizer
• add API documentation

Other changes (bug fixes, small refactors)

• wrapper to the module (not __init__) in hubconf.py
• add a citation to README.rst

Notes

to v2.1.1