[BUGFIX] fix valid candidates issue

[liuzh47]

Description

Fix the dynamic mask sampling strategy. Fix Issue #1321

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

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

cc @dmlc/gluon-nlp-team

[codecov]

Codecov Report

Merging #1323 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1323      +/-   ##
==========================================
+ Coverage   84.14%   84.23%   +0.08%     
==========================================
  Files          42       42              
  Lines        6397     6426      +29     
==========================================
+ Hits         5383     5413      +30     
+ Misses       1014     1013       -1     

Impacted Files	Coverage Δ
src/gluonnlp/data/tokenizers.py	77.67% <0.00%> (-0.03%)	⬇️
src/gluonnlp/models/transformer.py	98.93% <0.00%> (-0.01%)	⬇️
setup.py	0.00% <0.00%> (ø)
src/gluonnlp/data/sampler.py	96.55% <0.00%> (+0.32%)	⬆️
src/gluonnlp/utils/misc.py	52.53% <0.00%> (+0.63%)	⬆️
src/gluonnlp/sequence_sampler.py	87.34% <0.00%> (+0.78%)	⬆️
conftest.py	85.29% <0.00%> (+1.16%)	⬆️

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[sxjscience]

@liuzh91 Would you try to fix this along with the other Gumbel-topk issue? (So there is no need for me to merge twice).

[liuzh47]

@liuzh91 Would you try to fix this along with the other Gumbel-topk issue? (So there is no need for me to merge twice).

OK, I'll work on that