Make NeMo-Curator installable in non GPU environments

[ayushdg]

Right now NeMo-Curator has a mix of both CPU and GPU features. However it requires a cuda GPU to be present since the GPU dependencies are imported during library import and that ends up looking for cuda drivers.

Proposed Solution:

1. Move GPU accelerated versions of modules to an optional import that is not a requirement during library import.
2. Create an optional dependency while installing NeMo-Curator (maybe something like nemo-curator[cuda12x] following a similar convention to libraries like spacy/cupy/cuDF etc.

[ayushdg]

Completed via #27. pip install . will only install CPU dependencies and return placeholder objects for GPU libraries that throw an error if used. pip install "--extra-index-url https://pypi.nvidia.com ".[cuda12x]" will now pull in the GPU dependencies and relevant packages.