Change non expressive variable name on shared files

[rafaelteodosio]

Proposed changes:

• Change non expressive variables names on the files .../shared/core/training_data/visualization.py , .../shared/core/slots.py, .../shared/nlu/training_data/message.py.

• Resolve: #8520

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[sara-tagger]

Thanks for submitting a pull request 🚀 @dakshvar22 will take a look at it as soon as possible ✨

[dakshvar22]

Hi @rafaelteodosio , sorry for the delay. Are you still interested in taking this up? If yes, could you please merge the latest main branch into your branch so that we can start the review? 🙏

[stale]

This PR has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[ancalita]

@rafaelteodosio I can see that the development branch was deleted, as a result i will close this PR.
If you wish to continue work on this issue, please open a new PR.