spectrogram and inverse spectrogram

[nihui]

• [ ] custom window
• [x] padding for center=True
• [x] padding pad_mode=reflect
• [x] winlen normalized
• [x] pnnx convert https://pytorch.org/audio/stable/generated/torchaudio.functional.spectrogram.html
• [x] pnnx convert https://pytorch.org/audio/stable/generated/torchaudio.functional.inverse_spectrogram.html
• [x] pnnx convert https://pytorch.org/audio/stable/generated/torchaudio.transforms.Spectrogram.html
• [x] pnnx convert https://pytorch.org/audio/stable/generated/torchaudio.transforms.InverseSpectrogram.html
• [x] pnnx fuse stft pattern
• [x] pnnx ncnn stft test
• [x] pnnx ncnn spectrogram test
• [x] pnnx ncnn inverse spectrogram test
• [x] doc
• [x] coverage

notes

• only supports hann, hamming and all-one window
• inverse spectrogram does not support length parameter
• spectrogram always returns torch.view_as_real(out) as ncnn does not support complex typed mat yet
• inverse spectrogram always accepts torch.view_as_complex(in) as ncnn does not support complex typed mat yet

[tencent-adm]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.49246% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.99%. Comparing base (6a466ef) to head (68b3816). Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layer/inversespectrogram.cpp	97.97%	2 Missing :warning:
src/layer/spectrogram.cpp	99.00%	1 Missing :warning:

:exclamation: There is a different number of reports uploaded between BASE (6a466ef) and HEAD (68b3816). Click for more details.

HEAD has 31 uploads less than BASE

| Flag | BASE (6a466ef) | HEAD (68b3816) | |------|------|------| ||32|1|

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5779 +/- ## =========================================== - Coverage 95.00% 89.99% -5.02% =========================================== Files 795 267 -528 Lines 271196 79032 -192164 =========================================== - Hits 257640 71122 -186518 + Misses 13556 7910 -5646 ```

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