[TTS] Spectrogram Enhancer: support arbitrary input length
# What does this PR do ?

Make the spectrogram enhancer work on arbitrary length spectrograms irrespective of lengths during training. This a convolutional model (=locally-connected network), really no reason why it should not support arbitrary lengths

Original StyleGAN 2 architecture starts from a fixed-size noise-initialized tensor and generate images. To support arbitrary length spectrograms, the architecture was modified to start from a rather long tensor and chop off part of necessary length early on. This was not a good decision.

This PR makes the move from a long starting tensor to a tensor with a fixed length of 1.

**Collection**: TTS

# Changelog 
- Change length of the starting ("initial") tensor in the enhancer to 1
- Remove `max_spectrogram_length` from the model and the config

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**PR Type**:
- [ ] New Feature
- [x] Bugfix
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# Additional Information
* Fixes #6007