Python entities of Python API to NVBench should have docstrings

[oleksandr-pavlyk]

Presently, all doc-strings are collected in __init__.pyi

In [1]: import cuda.bench as bench

In [2]: bench.State.__doc__ is None
Out[2]: True

In [3]: bench.Launch.__doc__ is None
Out[3]: True

Even though information is there and can be extracted using ast package, it is unwieldy.

Doc strings should be moved to the source of the package, and be available as __doc__ attributes of classes, methods and functions defined in the package.