Impulse perturbation normalization typo
As this is my first issue, I MUST thank anyone contributing to this great repository, so I do :).

Without any context, [this](https://github.com/NVIDIA/NeMo/blob/50f16ba4f445e312b38b75a93881b9db2a10791c/nemo/collections/asr/parts/preprocessing/perturb.py#L377) normalization line seems wrong (first `min` should be `np.mean`).
`impulse_norm = (impulse.samples - min(impulse.samples)) / (max(impulse.samples) - min(impulse.samples))`

This totally changes the RIR signal and adds an overlapping echo to the output. Seems like nobody is using this augmentation :) or they didn't manually check the outputs.

An example RIR file from [BUT reverb dataset](https://speech.fit.vutbr.cz/software/but-speech-fit-reverb-database) is attached for anyone like myself who seeks a RIR example to check the augmentation.
[rir_1sec.zip](https://github.com/NVIDIA/NeMo/files/11230838/rir_1sec.zip)