key, value store abstraction

[seunghwak]

• create an abstract class to store (key, value) pairs and support searching with keys.
  • The abstract class can either use binary search or hash table (NVIDIA cuCollection) for search.
  • If value type is not directly supported by cuCollection (e.g. if not bitwise comparable or trivially copyable), we store key, value_index pairs in a cuCollection object and use the retrieved value_index to find a value.
• Update cugraph C++ code and per_v_transform_reduce_dst_key_aggregated_outgoing_e to use this abstraction (the latter is necessary to support (key, value) pairs with thrust::tuple type values; this is necessary for Leiden).

[seunghwak]

@naimnv Please review this, and now the per_v_transform_reduce_dst_key_aggregated_outoing_e primitive is updated to support thrust::tuple type value.

[ChuckHastings]

@gpucibot merge