[config] Expose get_total_num_hidden_layers() in ModelConfig

[ptovam]

This PR introduces get_total_num_hidden_layers() as a standalone helper in ModelConfig.
The logic previously lived inside get_layers_start_end_indices().

Motivation:
Expose the model’s total hidden-layer count through a clean, reusable API.

[gemini-code-assist]

Code Review

This pull request correctly refactors the logic for determining the total number of hidden layers into a new helper method, get_total_num_hidden_layers, within ModelConfig. This is a good change as it encapsulates complex, model-specific logic into a dedicated, reusable function, improving code clarity. The update to use a string forward reference for the ParallelConfig type hint is also a good practice to prevent potential circular imports. I have one suggestion to improve the maintainability of the newly extracted method.

[yewentao256]

LGTM, thanks for the work!

[ptovam]

Thank @yewentao256!
Pushed a small update to align the type-hint style with the rest of the file (removed the string annotation).