Update loss_func.py

[omkarcharatkar]

added Root Mean Squared error function.

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Brief description of what is fixed or changed

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[rohansingh9001]

@omkarcharatkar while the code LGTM, it seems that there are issues with lint in your PR. Please look at the automated tests for this PR and check for the lint errors.

We use PEP8 guidelines very strictly like most other python repositories and use flake8 to check for lint errors.

You can run such checks yourself by installing flake8 and running it in your python code repository.

python -m pip install flake8

In the code repository run -
flake8 .