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

---

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.

[david-cortes-intel]

This will also be set in other types of estimators - for example:
https://scikit-learn.org/stable/modules/generated/sklearn.multiclass.OneVsRestClassifier.html

Perhaps it could check if the first entry there is of tree type.

Also what about cuML? I guess it could be done by calling as_sklearn():
https://docs.rapids.ai/api/cuml/nightly/pickling_cuml_models/#Converting-Between-cuML-and-scikit-learn-Models