[BartTokenizerFast] add BartTokenizerFast in AutoTokenizer

[patil-suraj]

This PR adds BartTokenizerFast in AutoTokenizer.
@sshleifer

[codecov]

Codecov Report

Merging #6464 into master will decrease coverage by 2.57%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6464      +/-   ##
==========================================
- Coverage   79.91%   77.33%   -2.58%     
==========================================
  Files         153      153              
  Lines       28005    28005              
==========================================
- Hits        22379    21657     -722     
- Misses       5626     6348     +722     

Impacted Files	Coverage Δ
src/transformers/tokenization_auto.py	97.72% <100.00%> (ø)
src/transformers/optimization.py	25.55% <0.00%> (-70.00%)	⬇️
src/transformers/modeling_tf_distilbert.py	34.11% <0.00%> (-63.30%)	⬇️
src/transformers/pipelines.py	26.26% <0.00%> (-53.69%)	⬇️
src/transformers/optimization_tf.py	33.33% <0.00%> (-24.33%)	⬇️
src/transformers/modeling_tf_auto.py	48.79% <0.00%> (-18.08%)	⬇️
src/transformers/modeling_auto.py	64.16% <0.00%> (-14.46%)	⬇️
src/transformers/data/processors/squad.py	13.76% <0.00%> (-14.38%)	⬇️
src/transformers/modeling_tf_gpt2.py	65.68% <0.00%> (-6.16%)	⬇️
src/transformers/modelcard.py	82.71% <0.00%> (-2.47%)	⬇️
... and 12 more

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[LysandreJik]

LGTM, thanks for the two PRs @patil-suraj