Add model card for singbert large

[zyuanlim]

Model card for singbert large, similar to singbert but bert large model.

[zyuanlim]

@JetRunner same model but bert large version, thank you!!

[codecov]

Codecov Report

Merging #6721 into master will increase coverage by 0.49%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6721      +/-   ##
==========================================
+ Coverage   79.00%   79.50%   +0.49%     
==========================================
  Files         156      156              
  Lines       28406    28406              
==========================================
+ Hits        22443    22583     +140     
+ Misses       5963     5823     -140     

Impacted Files	Coverage Δ
src/transformers/modeling_tf_xlnet.py	21.12% <0.00%> (-71.05%)	⬇️
src/transformers/modeling_tf_flaubert.py	24.53% <0.00%> (-63.81%)	⬇️
src/transformers/modeling_roberta.py	77.37% <0.00%> (-19.71%)	⬇️
src/transformers/modeling_utils.py	88.05% <0.00%> (+0.55%)	⬆️
src/transformers/modeling_tf_utils.py	86.64% <0.00%> (+1.95%)	⬆️
src/transformers/generation_tf_utils.py	86.46% <0.00%> (+2.75%)	⬆️
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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