[s2s] command line args for faster val steps

[sshleifer]

cc @patil-suraj

[codecov]

Codecov Report

Merging #6833 into master will decrease coverage by 1.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6833      +/-   ##
==========================================
- Coverage   80.02%   78.84%   -1.19%     
==========================================
  Files         157      157              
  Lines       28586    28586              
==========================================
- Hits        22876    22538     -338     
- Misses       5710     6048     +338     

Impacted Files	Coverage Δ
src/transformers/configuration_openai.py	34.28% <0.00%> (-62.86%)	⬇️
src/transformers/tokenization_albert.py	28.84% <0.00%> (-58.66%)	⬇️
src/transformers/modeling_openai.py	23.87% <0.00%> (-57.10%)	⬇️
src/transformers/trainer.py	13.21% <0.00%> (-40.45%)	⬇️
src/transformers/modeling_tf_distilbert.py	64.47% <0.00%> (-34.36%)	⬇️
src/transformers/training_args.py	66.66% <0.00%> (-25.00%)	⬇️
src/transformers/trainer_utils.py	64.44% <0.00%> (-20.00%)	⬇️
src/transformers/tokenization_dpr.py	53.15% <0.00%> (-4.51%)	⬇️
src/transformers/configuration_bart.py	90.00% <0.00%> (-4.00%)	⬇️
src/transformers/data/datasets/glue.py	83.58% <0.00%> (-2.99%)	⬇️
... and 15 more

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