Division by zero in loudness calculation

[DanTremonti]

🐛 Describe the bug

The following line in the functional method loudness results in nan value when the entire waveform is below the hardcoded loudness threshold value gamma_abs = -70. https://github.com/pytorch/audio/blob/69b2a0adc2ec03ab99990d7e8be3d4510438c148/src/torchaudio/functional/functional.py#L1627-L1631

An example case is while trying to find loudness of an ambient sound signal.

The threshold can probably be made configurable with mention in documentation. However, I as the method returns a LUFS value, I am unsure if a configurable threshold should be allowed. I am not very familiar with the algorithm yet, any suggestions/corrections to what I've said is most welcome.

Versions

Latest code in main branch.