Added A General Swish Activation Function inNeural Networks

[shivansh-bhatnagar18]

Describe your change:

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

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[tianyizheng02]

Thanks for your contribution, but could you modify neural_network/activation_functions/sigmoid_linear_unit.py rather than making a separate file for the general swish function? Since the SiLU function is just the swish function with a parameter of 1, I think it'd make much more sense if you could rename that file to swish.py and generalize the existing function.

[shivansh-bhatnagar18]

@tianyizheng02 Sir, I added the general swish model in the Sigmoid Linear Unit Model and renamed it as 'swish.py'. Now in order to maintain learning through these algorithms, I didn't find it appealing enough to delete the existing SiLU function to let others know that it is a popular name for the swish function of parameter 1. Hence I just added both functions. Could you please look into this and merge my PR?

[tianyizheng02]

Thanks for your contribution!