Move LogitsDecoder into trt_helper.py and fix docstrings

[zhiqwang]

• Fix example in PredictorTRT
• Fix docstrings for PredictorTRT
• Move LogitsDecoder into trt_helper.py

[codecov]

Codecov Report

Merging #256 (b61ddc1) into main (902984d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #256   +/-   ##
=======================================
  Coverage   93.62%   93.62%           
=======================================
  Files          11       11           
  Lines         675      675           
=======================================
  Hits          632      632           
  Misses         43       43           

Flag	Coverage Δ
unittests	93.62% <ø> (ø)

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