Fix metrics for SE tutorial

[anteju]

What does this PR do ?

This PR fixes a bug in SE tutorial caused by initialization of metrics for audio-to-audio models.

Collection: ASR, Tutorials

Changelog

• Updated metric initialization
• Small fixes in the tutorial notebook

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• New Feature
• Bugfix
• Documentation

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Additional Information

• Related to # (issue)

[nithinraok]

LGTM