[ASR] Configurable metrics for audio-to-audio + removed experimental decorators
Signed-off-by: Ante Jukić <ajukic@nvidia.com>

# What does this PR do ?

This PR adds an option to configure metrics for `AudioToAudio` models.

**Collection**: ASR

# Changelog 
- Added `AudioMetricWrapper` to support batches with examples with variable input lengths
- Added unit tests
- Added example of using metrics in `multichannel_enhancement.yaml`
- Removed experimental decorators

# Usage
Metrics for `val` and `test` can be configured by adding `model.metrics`, for example

```
model:

  ...

  metrics:
    val:
      sdr: # output SDR
        _target_: torchmetrics.audio.SignalDistortionRatio
    test:
      sdr_ch0: # SDR on output channel 0
        _target_: torchmetrics.audio.SignalDistortionRatio
        channel: 0
```

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- [x] Did you write any new necessary tests?
- [ ] Did you add or update any necessary documentation?
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**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

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