Add docstring to `MultiModelTrainer` methods

[tizianocitro]

Issue must include at least one label from (bug, chore, docs, feature, refactoring)

Is your feature request related to a problem? Please describe.

I want docstrings for all methods in MultiModelTrainer.

[github-actions]

This issue is missing a required label.

Please add at least one of:

• feature
• fix
• chore
• refactoring

Current labels: docs, training

[github-actions]

This issue is missing a required label.

Please add at least one of:

• feature
• fix
• chore
• refactoring

Current labels: docs, training

[github-actions]

This issue is missing a required label.

Please add at least one of:

• feature
• fix
• chore
• refactoring

Current labels: docs, training