Add __setitem__ func to torchtext.experimental.vocab.Vocab

[zhangguanheng66]

On top of https://github.com/pytorch/text/pull/1027

Add __setitem__ func to torchtext.experimental.vocab.Vocab. A __delitem__ func is added as well.

[RuntimeError] if the token exists, a error message is sent out and ask users to delete the existing token from vocab.

Note: for the vocab, we don't support the assignment between a single index and multiple tokens.

[codecov[bot]]

Codecov Report

Merging #1113 (fe5ce92) into master (9053d95) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1113      +/-   ##
==========================================
+ Coverage   79.46%   79.52%   +0.06%     
==========================================
  Files          47       47              
  Lines        3175     3185      +10     
==========================================
+ Hits         2523     2533      +10     
  Misses        652      652              

Impacted Files	Coverage Δ
torchtext/experimental/vocab.py	100.00% <100.00%> (ø)

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