Improve leaf module interface (enable via config, relax matching criteria, add document, etc.)

[tohtana]

This PR improves the usability of the leaf module feature.

Here are the changes:

• Allow enabling the leaf module via both the DeepSpeed config and APIs.
• Relax matching criteria to support class-based matching.
• Support multiple ways of specifying the target module: class, class name (with or without package name), module name, or suffix.
• Add documentation to the training guide, including config snippets and explanations of default behavior.
• Add default classes (e.g., Mixtral, Qwen2/Qwen3) that automatically enable the leaf module feature. (Welcoming requests to add more classes)