Add typing.overload for convert_ids_tokens

[tamuhey]

The annotation of convert_ids_tokens is not sufficient. When ids are List[str] and str, the return type are always List[int] and int respectively.
It can be solved with typing.overload

[codecov]

Codecov Report

Merging #6637 into master will increase coverage by 0.80%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #6637      +/-   ##
==========================================
+ Coverage   79.47%   80.28%   +0.80%     
==========================================
  Files         156      156              
  Lines       28245    28251       +6     
==========================================
+ Hits        22448    22681     +233     
+ Misses       5797     5570     -227     

Impacted Files	Coverage Δ
src/transformers/tokenization_utils.py	89.84% <71.42%> (-0.56%)	⬇️
src/transformers/modeling_tf_flaubert.py	24.53% <0.00%> (-63.81%)	⬇️
src/transformers/modeling_xlnet.py	60.81% <0.00%> (-22.62%)	⬇️
src/transformers/modeling_roberta.py	77.37% <0.00%> (-19.71%)	⬇️
src/transformers/modeling_transfo_xl_utilities.py	52.98% <0.00%> (-13.44%)	⬇️
src/transformers/modeling_transfo_xl.py	67.17% <0.00%> (-12.53%)	⬇️
src/transformers/tokenization_xlm_roberta.py	84.52% <0.00%> (-10.72%)	⬇️
src/transformers/tokenization_utils_base.py	90.24% <0.00%> (-3.53%)	⬇️
src/transformers/generation_tf_utils.py	86.71% <0.00%> (+0.25%)	⬆️
src/transformers/file_utils.py	82.41% <0.00%> (+0.25%)	⬆️
... and 5 more

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[julien-c]

LGTM but will let others chime in

[LysandreJik]

Cool, like this a lot!

[tamuhey]

@LysandreJik What else should I do to merge this PR?