Change nvtext::load_vocabulary_file to return a unique ptr

[davidwendt]

Reference #5868

This PR changes the nvtext::load_vocabulary_file to return a unique-pointer to make it easier to manage in Python/Cython class object. The original signature returned a flat structure that contained unique-pointers which would make it difficult to copy and manage.

The corresponding gtests and gbenchmarks were updated for this API change.

[jrhemstad]

This is a code smell. There is rarely a reason to return a shared_ptr from a function. It should likely return a unique_ptr and if someone desires a shared_ptr they can trivially make it into one.

[kkraus14]

This is a code smell. There is rarely a reason to return a shared_ptr from a function. It should likely return a unique_ptr and if someone desires a shared_ptr they can trivially make it into one.

Just commenting that from the Python side we can handle a unique_ptr or a shared_ptr so defer to libcudf semantics here.

[codecov]

Codecov Report

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[codereport]

lgtm

[davidwendt]

@gpucibot merge