Add Average Callback

[Alexis-IMBERT]

What does this PR do?

Fixes #20617
Create Callback class that get in parameter : the metrics to smooth, the name of the smoothed metric and the callable function to use to smooth the metric, the window, (how many previous values to consider).
The callable function could be mean by default and window set to 5 by default.

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📚 Documentation preview 📚: https://pytorch-lightning--20618.org.readthedocs.build/en/20618/

[Alexis-IMBERT]

Please, don't hesitate to give information on things to add or change