Add inverse sqrt learning rate scheduler (#21495)

* added inverse sqrt lr scheduler

* Updated get_scheduler in src/transformers/optimization.py

* Updated src/transformers/__init__.py

* Added inverse sqrt lr scheduler test

* Updated docs/source/en/main_classes/optimizer_schedules.mdx

* Ran style and quality scripts

* Fix get_inverse_sqrt_schedule docstring

* Comment implementation URL

docs/source/en/main_classes/optimizer_schedules.mdx

@@ -60,6 +60,8 @@ The `.optimization` module provides:
 
 [[autodoc]] get_polynomial_decay_schedule_with_warmup
 
+[[autodoc]] get_inverse_sqrt_schedule
+
 ### Warmup (TensorFlow)
 
 [[autodoc]] WarmUp