update spectral clustering method
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Is it added by accident? @nithinraok  import guard this and warn / error out if the user doesn't have pyamg so that we can remove it from the requirements list  Thanks @DXist for the question. May I know your environment setup? Like Mac/Linux and conda or with pip. Because first gcc is required for conda installation. https://docs.conda.io/projects/conda-build/en/latest/resources/compiler-tools.html#compiler-packages 
 @nithinraok actually I use [miniforge](https://github.com/conda-forge/miniforge) and build inference and train Docker images using the [Dockerfile](http://ix.io/3mJ3). I get minimal images without build requirements. `base` target image is more compact then the image based on nvcr.io Pytorch. I could install pyamg from conda-forge but I haven't found any usages of pyamg in the code and decided to comment in this issue. pyamg is used as a backend, while computing embedding clusterers using sklearn. So its only useful for speaker diarization tasks. 
In NeMo we place speech classification, speaker dirarization, and of course speech recognition as part of asr collection, and hence the requirement.  Thank you for clarification! @nithinraok is it possible to have both spectral and COS clustering to pass as arguments to the diarizer. I have noticed that with the recent update to COS the DER has gone up from 0 to 0.4 for the an4.wav example file used in the demo notebooks 
This PR :
* Adds new NMSE spectral clustering method
* Updates Docs to include dataset creation for any dataset, with special mentions for AMI and CH109. 
* Adds ability to diarize for unknown number of speakers/session with very good accuracies 
* Replace SpectralCluster from requirements with pyamg (math computing repo) 

Signed-off-by: nithinraok <nithinrao.koluguri@gmail.com>