Resolve #40: Provide option to freeze the embedding parameters

[gpengzhi]

resolve #40

[codecov[bot]]

Codecov Report

Merging #271 into master will increase coverage by 0.05%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #271      +/-   ##
=========================================
+ Coverage   83.04%   83.1%   +0.05%     
=========================================
  Files         195     195              
  Lines       15300   15316      +16     
=========================================
+ Hits        12706   12728      +22     
+ Misses       2594    2588       -6

Impacted Files	Coverage Δ
texar/torch/modules/embedders/embedders.py	91.66% <ø> (ø)	:arrow_up:
texar/torch/modules/embedders/embedder_utils.py	100% <ø> (ø)	:arrow_up:
texar/torch/modules/embedders/embedder_base.py	94.11% <ø> (ø)	:arrow_up:
...exar/torch/modules/embedders/position_embedders.py	90.43% <ø> (+5.21%)	:arrow_up:
texar/torch/modules/embedders/embedders_test.py	98.11% <100%> (+0.33%)	:arrow_up:

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[huzecong]

The changes look good to me! Although not related to this PR, do you think you can add a few test cases to cover the not-yet-covered branches in position_embedders.py?

[gpengzhi]

The changes look good to me! Although not related to this PR, do you think you can add a few test cases to cover the not-yet-covered branches in position_embedders.py?

More unit tests added.