Adding triplet margin loss function

[ayushsingh11]

Hi all, this PR is in reference to issue #2200. I have cherry-picked commits from issue #2208 and added minor changes according to the discussion on it. Please review it and inform if any changes are required.

2 matrices are taken as input :

1. input - It consists of concatenated anchor and positive samples.
2. target - It consists of negative samples.

[mlpack-bot]

Thanks for opening your first pull request in this repository! Someone will review it when they have a chance. In the mean time, please be sure that you've handled the following things, to make the review process quicker and easier:

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Thank you again for your contributions! 👍

[ayushsingh11]

Thanks @NippunSharma . Added your changes and a few more in the implementation of Boost. Now the build was successful and test cases are also passing.

[ayushsingh11]

Thanks @zoq , made the changes you suggested. Hoping everything else is fine.

[zoq]

No more comments from my side, thanks for picking this up.

[mlpack-bot]

Second approval provided automatically after 24 hours. 👍

[mlpack-bot]

Hello there! Thanks for your contribution. I see that this is your first contribution to mlpack. If you'd like to add your name to the list of contributors in COPYRIGHT.txt and you haven't already, please feel free to push a change to this PR---or, if it gets merged before you can, feel free to open another PR.

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[shrit]

Everything looks good to me, thanks for the contributions 👍

[ayushsingh11]

Thanks @shrit for the review.

[shrit]

@ayushsingh11 There is a conflict in the history file, can you resolve the conflict so I can merge this pull request?

[ayushsingh11]

@shrit resolved the conflict.

[zoq]

Thanks again for the contribution.