Fix zero-shot pipeline single seq output shape

[joeddav]

Fixes zero shot pipelines bug that returns sequence as a list rather than a str when a single sequence is passed as a list.

[codecov]

Codecov Report

Merging #6104 into master will decrease coverage by 1.54%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6104      +/-   ##
==========================================
- Coverage   77.77%   76.23%   -1.55%     
==========================================
  Files         146      146              
  Lines       26325    26325              
==========================================
- Hits        20474    20068     -406     
- Misses       5851     6257     +406     

Impacted Files	Coverage Δ
src/transformers/pipelines.py	78.50% <ø> (ø)
src/transformers/data/data_collator.py	19.82% <0.00%> (-77.59%)	⬇️
...c/transformers/modeling_tf_transfo_xl_utilities.py	9.90% <0.00%> (-76.24%)	⬇️
src/transformers/modeling_tf_transfo_xl.py	17.22% <0.00%> (-72.24%)	⬇️
src/transformers/tokenization_xlm.py	16.26% <0.00%> (-66.67%)	⬇️
...rc/transformers/data/datasets/language_modeling.py	34.69% <0.00%> (-57.15%)	⬇️
src/transformers/data/datasets/glue.py	50.74% <0.00%> (-35.83%)	⬇️
src/transformers/tokenization_bart.py	60.00% <0.00%> (-35.72%)	⬇️
src/transformers/modeling_tf_gpt2.py	65.42% <0.00%> (-29.91%)	⬇️
src/transformers/trainer_utils.py	60.00% <0.00%> (-25.72%)	⬇️
... and 10 more

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