PERF-#6710: Don't materialize index in _groupby_shuffle internal function

[anmyachev]

What do these changes do?

Length materialization is in most cases cheaper than index materialization.

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• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
• signed commit with git commit -s
• Resolves Don't materialize index in _groupby_shuffle internal function #6710
• tests added and passing
• module layout described at docs/development/architecture.rst is up-to-date

[dchigarev]

Length materialization is in most cases cheaper than index materialization.

It depends on which cache is available right now. If we're willing to do this kind of optimization, I would propose to add a __len__() method for ModinDataframe that would compute the length either using index or row lengths cache, depending on which information it has materialized right now

[anmyachev]

Length materialization is in most cases cheaper than index materialization.

It depends on which cache is available right now. If we're willing to do this kind of optimization, I would propose to add a __len__() method for ModinDataframe that would compute the length either using index or row lengths cache, depending on which information it has materialized right now

@dchigarev I added __len__.