Simplify package dependencies by removing unnecessary imports

[NeoChaos12]

I currently use NASLib as a backend for defining my own search space for a benchmark. I noticed while sharing my code that the following imports introduce a number of dependencies that are not strictly necessary in use cases where only a subset of all the search spaces defined by NASLib are to be used. Auto-importing these search spaces enforces the constraint that the dependencies of each and every search space must be satisfied in order to use any single one of the search spaces. Is this truly necessary? For reference, I am currently using only the NASBench-201 search space.

NASLib/naslib/search_spaces/__init__.py

Lines 1 to 10 in afa98c3

	from .simple_cell.graph import SimpleCellSearchSpace
	from .darts.graph import DartsSearchSpace
	from .nasbench101.graph import NasBench101SearchSpace
	from .nasbench201.graph import NasBench201SearchSpace
	from .nasbenchnlp.graph import NasBenchNLPSearchSpace
	from .nasbenchasr.graph import NasBenchASRSearchSpace
	from .hierarchical.graph import HierarchicalSearchSpace
	from .transbench101.graph import TransBench101SearchSpace
	from .transbench101.api import TransNASBenchAPI

[georgiachanning]

Great issue. Have you had any success removing the packages and running without excess dependencies?