Standardized dependency installation for GitHub actions

[scap3yvt]

Fixes #920

Proposed Changes

• all dependency installations are happening at a single place
• includes Torch version updated #919, so marking this as draft until that is merged

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].

[github-actions]

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[scap3yvt]

I am unable to figure out how to save the output variable: https://github.com/scap3yvt/GaNDLF/blob/920-feature-standardize-dependency-installation-for-github-actions/.github/workflows/dependencies/action.yml#L8

Which is why subsequent steps that depend on this variable [ref] are not getting triggered.

Any help would be much appreciated.