Refactor the code related to loss computation

[szmazurek]

Fixes #854

Proposed Changes

• create a common interface for the loss functions
• refactor the codebase to use new loss implementations

Checklist

• CONTRIBUTING guide has been followed.
• PR is based on the current GaNDLF master .
• Non-breaking change (does not break existing functionality): provide as many details as possible for any breaking change.
• Function/class source code documentation added/updated (ensure typing is used to provide type hints, including and not limited to using Optional if a variable has a pre-defined value).
• Code has been blacked for style consistency and linting.
• If applicable, version information has been updated in GANDLF/version.py.
• If adding a git submodule, add to list of exceptions for black styling in pyproject.toml file.
• Usage documentation has been updated, if appropriate.
• Tests added or modified to cover the changes; if coverage is reduced, please give explanation.
• If customized dependency installation is required (i.e., a separate pip install step is needed for PR to be functional), please ensure it is reflected in all the files that control the CI, namely: python-test.yml, and all docker files [1,2,3].
• The logging library is being used and no print statements are left.

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[szmazurek]

@sarthakpati What is your opinion on the implementation of interface and example loss? When we agree on that I will proceed to port the rest.

[sarthakpati]

Should this PR not be going into https://github.com/szmazurek/GaNDLF/tree/dev ?

[szmazurek]

Should this PR not be going into https://github.com/szmazurek/GaNDLF/tree/dev ?

Could be, but as it is directly related to existing core code I tought that we can update directly here.

[sarthakpati]

This commit was done without proper git initialization and hence is getting flagged by the CLA bot. Can you rebase this?