[WIP] Language modeling example for TF Trainer

[ydshieh]

To support language modeling for TF Trainer.

[codecov]

Codecov Report

Merging #6955 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6955      +/-   ##
==========================================
- Coverage   80.02%   80.00%   -0.02%     
==========================================
  Files         161      161              
  Lines       30120    30120              
==========================================
- Hits        24104    24098       -6     
- Misses       6016     6022       +6     

Impacted Files	Coverage Δ
src/transformers/generation_tf_utils.py	85.21% <0.00%> (-1.26%)	⬇️
src/transformers/file_utils.py	82.41% <0.00%> (-0.26%)	⬇️
src/transformers/data/data_collator.py	92.30% <0.00%> (ø)

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[stale]

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