Added Scaled Exponential Linear Unit Activation Function

[AdarshAcharya5]

Added Scaled Exponential Linear Unit Activation (SELU) Function under TheAlgorithms/Python/neural_network/activation_functions. Description of SELU taken from reference link provided in the top comment section in scaled_exponential_linear_unit.py
Fixes #9010

• Add an algorithm?

Checklist:

• I have read CONTRIBUTING.md.
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• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
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[AdarshAcharya5]

@tianyizheng02 Thanks for your reply in the discussion section!. All checks passed now.

[tianyizheng02]

Is there a reason why _lambda has an underscore? Is the user not meant to change this coefficient?

[AdarshAcharya5]

Yes, firstly lambda is a reserved keyword in python. Secondly both alpha and lambda are fixed constants.

[AdarshAcharya5]

The user can change these values, that may yield slightly different behaviour from the function, but the default values given to them are alpha: float = 1.6732, _lambda: float = 1.0507.

[tianyizheng02]

Yes, firstly lambda is a reserved keyword in python.

Oh yeah, duh 🤦

Could you rename the variable to something like lambda_ instead? Having an underscore at the start of a variable name generally signifies that the user isn't supposed to use it.

[AdarshAcharya5]

Ah. Yes for sure, I'll get it done right away!

[AdarshAcharya5]

Done