opt: decouple OptimizationState from get_momentum and renames

[iblislin]

Blocker: #394

renames:

• get_momentum -> getmomentum
• get_momentum_scheduler -> getmomsched
• Momentum.Fixed.momentum -> Momentum.Fixed.μ

[iblislin]

Brach diff here: https://github.com/dmlc/MXNet.jl/compare/ib/nadam-sched...ib/opt-getmomentum

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@9d60663). Click here to learn what that means. The diff coverage is 83.33%.

@@            Coverage Diff            @@
##             master     #395   +/-   ##
=========================================
  Coverage          ?   70.75%           
=========================================
  Files             ?       27           
  Lines             ?     2055           
  Branches          ?        0           
=========================================
  Hits              ?     1454           
  Misses            ?      601           
  Partials          ?        0

Impacted Files	Coverage Δ
src/optimizers/sgd.jl	100% <100%> (ø)
src/optimizers/nadam.jl	100% <100%> (ø)
src/optimizer.jl	71.42% <78.57%> (ø)

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[iblislin]

covered in #396