Update decoder.py

[sw005320]

added a one-line comment to further clarify the intentional dropout

[sw005320]

CI has strange behaviors.
I'll close this PR.

[codecov]

Codecov Report

Merging #4322 (3bb96f1) into master (40202fc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4322   +/-   ##
=======================================
  Coverage   81.66%   81.66%           
=======================================
  Files         458      458           
  Lines       40086    40086           
=======================================
  Hits        32736    32736           
  Misses       7350     7350           

Flag	Coverage Δ
test_integration_espnet1	67.13% <ø> (ø)
test_integration_espnet2	50.12% <ø> (ø)
test_python	68.25% <ø> (ø)
test_utils	24.45% <ø> (ø)

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Impacted Files	Coverage Δ
espnet/nets/pytorch_backend/tacotron2/decoder.py	99.09% <ø> (ø)

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