Added doctest, docstring and typehint for sigmoid_function & cost_function (#10828)

* Added doctest for sigmoid_function & cost_function

* Update logistic_regression.py

* Update logistic_regression.py

* Minor formatting changes in doctests

* Apply suggestions from code review

* Made requested changes in logistic_regression.py

* Apply suggestions from code review

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Co-authored-by: Tianyi Zheng <[email protected]>