fix validation error for models like z-image that do not use dynamic shift

[bghira]

This pull request makes a targeted change to the logic for determining whether to apply a dynamic shift when getting the timestep schedule in validation.py. The update tightens the condition so that both flow_schedule_auto_shift and use_dynamic_shifting must be enabled, rather than either one.

• Logic correction for dynamic shift application:
  • In get_timestep_schedule, the condition now requires both self.config.flow_schedule_auto_shift and dynamic_shift (from noise_scheduler.config.use_dynamic_shifting) to be true before applying the dynamic shift logic, instead of just one or the other. This prevents unintended application of dynamic shifting when only one flag is set.