update speaker collate functions
Signed-off-by: nithinraok <nithinrao.koluguri@gmail.com>

This PR 
* removes time-length and shift-length concept for training speaker label models. 
> This assumes all the samples present in training manifest must adhere to max of 8 secs. (3 sec is ideal) 

* removes sliced_seq_collate_fn : Since we introduced segmentation for creating manifest files hence this collate func is no longer required now, and existing seq_collate_fn can be used for getting speaker embeddings for diarization as well. 

