Normalize loudness of the generated audio (#1795)

* Normalize loudness of the generated audio.

* Lints.

* One more lint.

* Avoid running the bs1770 tests.

* Another attempt at discarding doc comments.

* Also normalize the loudness in the encodec example.

candle-examples/src/audio.rs

@@ -0,0 +1,29 @@
+use candle::{Result, Tensor};
+
+// https://github.com/facebookresearch/audiocraft/blob/69fea8b290ad1b4b40d28f92d1dfc0ab01dbab85/audiocraft/data/audio_utils.py#L57
+pub fn normalize_loudness(
+    wav: &Tensor,
+    sample_rate: u32,
+    loudness_compressor: bool,
+) -> Result<Tensor> {
+    let energy = wav.sqr()?.mean_all()?.sqrt()?.to_vec0::<f32>()?;
+    if energy < 2e-3 {
+        return Ok(wav.clone());
+    }
+    let wav_array = wav.to_vec1::<f32>()?;
+    let mut meter = crate::bs1770::ChannelLoudnessMeter::new(sample_rate);
+    meter.push(wav_array.into_iter());
+    let power = meter.as_100ms_windows();
+    let loudness = match crate::bs1770::gated_mean(power) {
+        None => return Ok(wav.clone()),
+        Some(gp) => gp.loudness_lkfs() as f64,
+    };
+    let delta_loudness = -14. - loudness;
+    let gain = 10f64.powf(delta_loudness / 20.);
+    let wav = (wav * gain)?;
+    if loudness_compressor {
+        wav.tanh()
+    } else {
+        Ok(wav)
+    }
+}