add new lr_scheduler_type: step_lr
Author: Xingfu-Yi
Pull request: yes

# What does this PR do?

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Added a new learning rate scheduler: `step_lr`. This was implemented through the `TYPE_TO_SCHEDULER_FUNCTION get_step_lr`, based on the existing class `torch.optim.lr_scheduler.StepLR`. It retains the same parameters as other functions and uses `num_training_steps` to represent `step_size`, ensuring consistency.



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Comments:
Hi @Xingfu-Yi, thanks for opening a PR! 

Could you provide some more context around this addition - ideally links to related feature requests, paper and research? 

cc @younesbelkada @muellerzr 

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