Calculate number of nodes in the tree estimators

[avolkov-intel]

Description

• Automatically report total number of tree nodes (n_nodes) for Random Forest and Extra Trees estimators after training
• Register n_nodes as an indifferent metric in the report so it's compared across libraries rather than treated as a grouping parameter

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Checklist:

Completeness and readability

• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation to reflect the changes or created a separate PR with updates and provided its number in the description, if necessary.
• Git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details).
• I have resolved any merge conflicts that might occur with the base branch.

Testing

• I have run it locally and tested the changes extensively.
• All CI jobs are green or I have provided justification why they aren't.
• I have extended testing suite if new functionality was introduced in this PR.