[TTS] Fix FastPitch data prep tutorial

[rlangman]

What does this PR do ?

Update TTS tutorial to be consistent with recent code changes.

Merge to r1.21.0 branch.

Collection: [TTS]

Changelog

• Update lower_case and batch_size arguments examples for preprocess_text.py
• Add dataset name to path of logged audio.

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PR Type:

• New Feature
• Bugfix
• Documentation