Add an NVIDIA DALI implementation of AudioToMelSpectrogramPreprocessor
This pull request **introduces 7 alerts** when merging d95d5d521e8ce34b24793e8587a66c1eaeb1d925 into bee43e840837d7e30e17a65b972e9fe89ecb9ca5 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-37ade276ee5798ce168ea17564133c5501e5e918)

**new alerts:**

* 4 for Unused import
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* 1 for Unused local variable This pull request **introduces 1 alert** when merging 9002b4daff2956fda9e65035b2cb145626887f76 into bee43e840837d7e30e17a65b972e9fe89ecb9ca5 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-0767ff2f37952d52429a366b751eee2a01f544b0)

**new alerts:**

* 1 for Unreachable code This pull request **introduces 1 alert** when merging ff4ac0444bc4feaf108dab29e65cd22fe28db159 into bee43e840837d7e30e17a65b972e9fe89ecb9ca5 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-fd490f27420186d5c834ebc7dc7115a88adb9302)

**new alerts:**

* 1 for Unreachable code Closed for the time being while other approaches are tested
Adds an option `use_dali` to `AudioToMelSpectrogramPreprocessor` to choose between the native torch implementation of the audio preprocessing pipeline or a DALI based implementation.

Adds unit tests to verify that the results produced by both implementations are equivalent.

Removes code related with previous attempts to integrate with NVIDIA DALI. 