[FEATURE] Add length normalized metrics for machine translation tasks

[liuzh47]

Description

• Add length normalized loss metric for machine translation task.
• Test cases are included for the new metrics.
• Remove Gluonnlp EvalMetric class

Fixes #1094

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

[codecov]

Codecov Report

Merging #1095 into master will increase coverage by 0.44%.
The diff coverage is 90.9%.

@@            Coverage Diff            @@
##           master   #1095      +/-   ##
=========================================
+ Coverage   88.26%   88.7%   +0.44%     
=========================================
  Files          67      68       +1     
  Lines        6312    6285      -27     
=========================================
+ Hits         5571    5575       +4     
+ Misses        741     710      -31

Impacted Files	Coverage Δ
src/gluonnlp/metric/__init__.py	100% <100%> (ø)	⬆️
src/gluonnlp/metric/masked_accuracy.py	100% <100%> (+39.53%)	⬆️
src/gluonnlp/metric/length_normalized_loss.py	89.28% <89.28%> (ø)

[leezu]

Please merge master due to #1096

[mli]

Job PR-1095/4 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1095/4/index.html

[mli]

Job PR-1095/5 is complete.
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[mli]

Job PR-1095/6 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1095/6/index.html

[mli]

Job PR-1095/7 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1095/7/index.html

[mli]

Job PR-1095/8 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1095/8/index.html