Fix ONNX test_quantize unittest

[mfuntowicz]

quantize was catching exception silently, it now let the exception be catched by the caller to avoid dead path.

Also, introduces extras dependencies ["onnxruntime"] required to quantize a model and installs the extra when running slow tests.

[codecov]

Codecov Report

Merging #6716 into master will increase coverage by 1.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6716      +/-   ##
==========================================
+ Coverage   78.98%   80.01%   +1.03%     
==========================================
  Files         156      156              
  Lines       28398    28398              
==========================================
+ Hits        22429    22724     +295     
+ Misses       5969     5674     -295     

Impacted Files	Coverage Δ
src/transformers/modeling_tf_albert.py	21.47% <0.00%> (-69.44%)	⬇️
src/transformers/modeling_tf_flaubert.py	24.53% <0.00%> (-63.81%)	⬇️
src/transformers/tokenization_dpr.py	53.15% <0.00%> (-4.51%)	⬇️
src/transformers/modeling_utils.py	88.05% <0.00%> (+0.55%)	⬆️
src/transformers/modeling_tf_utils.py	87.29% <0.00%> (+2.60%)	⬆️
src/transformers/generation_tf_utils.py	86.46% <0.00%> (+5.01%)	⬆️
src/transformers/configuration_t5.py	96.42% <0.00%> (+10.71%)	⬆️
src/transformers/modeling_t5.py	83.83% <0.00%> (+12.21%)	⬆️
src/transformers/modeling_tf_t5.py	90.93% <0.00%> (+64.09%)	⬆️
src/transformers/modeling_tf_electra.py	98.95% <0.00%> (+73.82%)	⬆️

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[sgugger]

LGTM!

[sshleifer]

Lets see what happens!