[TTS] Spectrogram Enhancer: correct dim for length when loading data
# What does this PR do ?

Makes DataLoader for Spectrogram enhancer (#5565) use correct dimension to determine length of the spectrogram.

Currently it uses number of mel bands as length, so in practice enhancer trains on 80x80 patches, which is obviously suboptimal and a **bug**.

**Collection**: TTS

# Changelog 
- Fixes just one single dimension index

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**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

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# Additional Information
* Related to #6007 but there's more work to do