[Fix] NoisePerturbation for multi-channel audios
# What does this PR do ?

Fix the error caused by `noise_gain_db = min(data_rms - noise.rms_db - snr_db, self._max_gain_db)` when handling multi-channel audios. The error was that `min(np.array([1,2,3]), 4)` is invalid since `np.array([1,2,3]) < 4` is ambiguous. To fix it, we first check whether the audio is multi-channel, then we perform the `min()` operation for each channel if it's multi-channel.


**Collection**: ASR



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**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

