Refactor speech recognition example

[vandanavk]

Description

• Cleanup/simplify speech recognition example
• Refactor singleton usage
• Fix Python3 failures speech recognition example is not compatible with python3 #11042

Package used (Python/R/Scala/Julia):
Python 2 and 3

Build info (Required if built from source)

Compiler (gcc/clang/mingw/visual studio):

MXNet commit hash:
6a7bfe9

Related issues

#12024
PR #12291

[anirudhacharya]

@mxnet-label-bot [Example, Good First Issue]

[vishaalkapoor]

@vandanavk is the checked in PR the only change for this issue? If so, would you be able to close this issue off? Thanks!

[vandanavk]

The checked in PR fixes a few bugs but does not address the issues mentioned here. Since fixing the example would be a larger effort, based on comments from reviewers, I opened this issue for tracking.

[vandanavk]

PR #13068