Override get_vocab for fast tokenizer.

[mfuntowicz]

No description provided.

[codecov-commenter]

Codecov Report

Merging #4717 into master will decrease coverage by 1.23%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #4717      +/-   ##
==========================================
- Coverage   77.38%   76.14%   -1.24%     
==========================================
  Files         128      128              
  Lines       21071    21073       +2     
==========================================
- Hits        16305    16046     -259     
- Misses       4766     5027     +261     

Impacted Files	Coverage Δ
src/transformers/tokenization_utils.py	89.59% <50.00%> (+0.02%)	⬆️
src/transformers/modeling_longformer.py	18.70% <0.00%> (-74.83%)	⬇️
src/transformers/configuration_longformer.py	76.92% <0.00%> (-23.08%)	⬇️
src/transformers/modeling_t5.py	77.18% <0.00%> (-6.35%)	⬇️
src/transformers/file_utils.py	73.44% <0.00%> (-0.42%)	⬇️
src/transformers/modeling_bert.py	86.77% <0.00%> (-0.19%)	⬇️
src/transformers/data/processors/squad.py	56.68% <0.00%> (+28.02%)	⬆️

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