Returning Index instead of boolean in knuth_morris_pratt (kmp) function, making it compatible with str.find(). (#9083)

* Update knuth_morris_pratt.py - changed Boolean to Index

* Update knuth_morris_pratt.py

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* Update knuth_morris_pratt.py

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* Update knuth_morris_pratt.py

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* Update back_propagation_neural_network.py

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* Update strings/knuth_morris_pratt.py

* Update knuth_morris_pratt.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <[email protected]>