Add new AutoModel classes in pipeline

[patil-suraj]

This PR removes the about to be deprecated AutModelWithLMHead class and uses the new AutoModelForSeq2SeqLM, AutoModelForCausalLM and AutoModelForMaskedLM for translation, text-generation and fill-mask pipelines respectively.

Regrading issue #6060

@sgugger

[sgugger]

This is cleaner, thanks for tackling this!

[LysandreJik]

LGTM, thanks!

[codecov]

Codecov Report

Merging #6062 into master will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6062      +/-   ##
==========================================
+ Coverage   78.50%   78.67%   +0.17%     
==========================================
  Files         146      146              
  Lines       26251    26251              
==========================================
+ Hits        20609    20654      +45     
+ Misses       5642     5597      -45     

Impacted Files	Coverage Δ
src/transformers/pipelines.py	77.00% <100.00%> (ø)
src/transformers/modeling_tf_distilbert.py	64.90% <0.00%> (-33.90%)	⬇️
src/transformers/generation_tf_utils.py	83.95% <0.00%> (-1.51%)	⬇️
src/transformers/file_utils.py	81.19% <0.00%> (-0.30%)	⬇️
src/transformers/generation_utils.py	96.82% <0.00%> (-0.29%)	⬇️
src/transformers/modeling_auto.py	78.48% <0.00%> (+4.06%)	⬆️
src/transformers/modeling_tf_electra.py	95.53% <0.00%> (+69.51%)	⬆️

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