Added Swish Activation Function

[kausthub-kannan]

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• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

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[kausthub-kannan]

Hello,
the pre-commit is failing due to an issue with data_structures/circulra_linked_list.py

[tianyizheng02]

The swish function is already implemented in maths/sigmoid_linear_unit.py (though admittedly it should be moved to neural_network/activation_functions).