Display the number of trainable parameters in Trainer when lauching a training
Author: regisss
Pull request: yes

# What does this PR do?

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When launching a training with an instance of the `Trainer` class, a small recap is displayed with various pieces of information (total train batch size, total optimization steps, etc...).
This PR adds to this recap the number of trainable parameters of the model because this is not always mentioned in the model card or in the documentation and I think it is a valuable figure to have in all runs.


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Comments:
_The documentation is not available anymore as the PR was closed or merged._