Feature/optimizations

[belerico]

Summary

In this PR we have added some optimizations, in particular:

• The creation of the metric on the timer object is not needed when the timer is disabled
• In the MultiEncoder class the concatenation of empty tensors with dense ones has been removed
• We now let the user decide if he/she wants to run the test at the end of the training, true by default

Type of Change

Please select the one relevant option below:

• New feature (non-breaking change that adds functionality)

Checklist

Please confirm that the following tasks have been completed:

• I have tested my changes locally and they work as expected. (Please describe the tests you performed.)
• I have added unit tests for my changes, or updated existing tests if necessary.
• I have updated the documentation, if applicable.
• I have installed pre-commit and run locally for my code changes.

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