add image for specaugment docstring

[vincentqb]

Adding images to docstring to illustrate SpecAugment added in #285 as suggested by moto. This could look good online?

To regenerate:

import torchaudio

filename = "steam-train-whistle-daniel_simon.wav"
waveform, sample_rate = torchaudio.load(filename)
specgram = torchaudio.transforms.Spectrogram(n_fft=1054)(waveform)

augmentations = torchaudio.transforms.TimeMasking(time_mask_param=100)
augmented = augmentations(specgram[0, ...].log())
plt.imshow(augmented.numpy(), cmap='gray');
# plt.axis('off')
plt.savefig('specaugment_time.jpg')

augmentations = torchaudio.transforms.FrequencyMasking(freq_mask_param=100)
augmented = augmentations(specgram[0, ...].log())
plt.imshow(augmented.numpy(), cmap='gray');
# plt.axis('off')
plt.savefig('specaugment_freq.jpg')

[cpuhrsch]

I'd add to the description a line describing the motivation behind this change.

[vincentqb]

Image in docstring in pytorch, or here

[cpuhrsch]

I think it's fine if the docstring can still stand by itself and the images are supplemental

[mthrok]

I think it would look better with a colormap other than grayscale.

https://matplotlib.org/cmocean/

[cpuhrsch]

On the color scheme: I'd play with it, but make sure it's still visible for people with color blindness and related

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[mthrok]

Addressed by #1887