bypass validation scheduler setup for special models

[bghira]

This pull request introduces a new mechanism for models to indicate if they require special handling when setting up schedulers during validation. Several model classes now override a new method to signal that scheduler setup should be delegated to the model pipeline, and the validation logic is updated to respect this flag.

Scheduler setup improvements:

• Added a new method requires_special_scheduler_setup to the base model class (common.py), allowing models to specify if they need custom scheduler initialization.
• Updated the validation logic in validation.py to check requires_special_scheduler_setup and skip default scheduler setup if the method returns True.

Model-specific overrides:

• Implemented requires_special_scheduler_setup to return True for the following models, indicating that they handle scheduler setup internally:
  • ltxvideo (model.py)
  • ltxvideo2 (model.py)
  • Sana (model.py)
  • SanaVideo (model.py)
  • Wan (model.py)