Implemented gradient descent in C++

[Mysjkin]

Description

I have implemented a simple version of gradient descent in c++ for a univariate linear regression.

• Added a folder called Gradient descent in the machine learning folder
• Added a file called SimpleGradientDescent.cpp that implements gradient descent for a linear regression.
• Added a file called utility.h which are simple utility methods I used and can be used by others in other variants of gradient descent.
• Added a readme that documents the basic aspects of gradient descent optimisation.

Related Issue

This pr resolves #335
Please link to the issue here: #335

How Has This Been Tested? (Optional)

It has been tested by learning the weights of a univariate linear function.

Types of changes

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• Bug fix (change which fixes an issue with the algorithm)

• New Algorithm (non-breaking change which adds functionality)

• Documentation

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

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• My code follows the code style of this project.

• My change requires a change to the documentation.

• I have updated the documentation accordingly.

• I have read the CONTRIBUTING document.