Update experimental vectors without unk tensor

[zhangguanheng66]

Remove the unk tensor and allow users to add one if necessary.

[codecov[bot]]

Codecov Report

Merging #1065 (af6da94) into master (9053d95) will increase coverage by 0.03%. The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master    #1065      +/-   ##
==========================================
+ Coverage   79.46%   79.49%   +0.03%     
==========================================
  Files          47       47              
  Lines        3175     3185      +10     
==========================================
+ Hits         2523     2532       +9     
- Misses        652      653       +1     

Impacted Files	Coverage Δ
torchtext/experimental/vectors.py	90.52% <95.45%> (-0.07%)	:arrow_down:

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