use the correct add_start_docstrings

[stas00]

As discussed at #6940 (comment)
fixing to use the correct decorator add_start_docstrings

@sgugger

[codecov]

Codecov Report

Merging #7174 into master will decrease coverage by 0.40%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7174      +/-   ##
==========================================
- Coverage   80.71%   80.31%   -0.41%     
==========================================
  Files         169      169              
  Lines       32293    32293              
==========================================
- Hits        26066    25936     -130     
- Misses       6227     6357     +130     

Impacted Files	Coverage Δ
src/transformers/tokenization_marian.py	99.15% <100.00%> (ø)
src/transformers/tokenization_mbart.py	96.82% <100.00%> (ø)
src/transformers/tokenization_pegasus.py	95.23% <100.00%> (ø)
src/transformers/modeling_tf_electra.py	25.32% <0.00%> (-73.63%)	⬇️
src/transformers/configuration_bart.py	90.00% <0.00%> (-4.00%)	⬇️
src/transformers/data/data_collator.py	92.83% <0.00%> (-0.72%)	⬇️
src/transformers/modeling_tf_utils.py	87.01% <0.00%> (-0.33%)	⬇️
src/transformers/modeling_bart.py	93.77% <0.00%> (-0.17%)	⬇️
src/transformers/tokenization_dpr.py	57.65% <0.00%> (+4.50%)	⬆️
src/transformers/modeling_mobilebert.py	89.45% <0.00%> (+10.24%)	⬆️
... and 2 more

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