Improve ease of use of `.plot_ensemble_model()`

### Describe the issue linked to the documentation

Due to dependency requirements, `predictor.plot_ensemble_model()` is not easy to use.

https://github.com/awslabs/autogluon/blob/master/tabular/src/autogluon/tabular/predictor/predictor.py#L2659

### Suggest a potential alternative/fix

Refactor `predictor.plot_ensemble_model()` and make the dependency rely on a package that is installed by default in AutoGluon and works across Linux, Mac, and Windows. The refactor should also work in notebook environments.

Start at tabular/src/autogluon/tabular/predictor/predictor.py around line 2659 and inspect predictor.plot_ensemble_model(). Check how its current dependency is used, then verify the refactor with Linux, macOS, Windows, and notebook environments. Done means the method relies on a package installed by default and is easy to use across those environments.

Written by the indexing model from the issue text.