Adding dunder methods to cpp Vectors

[Nayef211]

Description

• Updated cpp Vector methods with dunder methods
• Now we can get and set elements in cpp Vectors class using [] operator
• Updated teardown method in tests to work with the new registry names

[codecov]

Codecov Report

Merging #852 into master will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #852      +/-   ##
==========================================
- Coverage   76.86%   76.85%   -0.02%     
==========================================
  Files          42       42              
  Lines        2944     2947       +3     
==========================================
+ Hits         2263     2265       +2     
- Misses        681      682       +1     

Impacted Files	Coverage Δ
torchtext/experimental/vectors.py	86.23% <80.00%> (-0.56%)	⬇️

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