Remove hard-coded pad token id in distilbert and albert

[monologg]

As the config adds pad_token_id attribute, padding_idx set the value of config.pad_token_id in BertEmbedding. ( PR #3793 )

But it seems that not only the config of Bert, but also that of DistilBert and Albert has pad_token_id. (Distilbert config, Albert config)

But in Embedding class of Distilbert and Albert, it seems that padding_idx is still hard-coded. So I've fixed those parts.

[codecov-io]

Codecov Report

Merging #3965 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3965      +/-   ##
==========================================
- Coverage   78.45%   78.44%   -0.02%     
==========================================
  Files         111      111              
  Lines       18518    18518              
==========================================
- Hits        14528    14526       -2     
- Misses       3990     3992       +2     

Impacted Files	Coverage Δ
src/transformers/modeling_albert.py	75.31% <100.00%> (ø)
src/transformers/modeling_distilbert.py	98.15% <100.00%> (ø)
src/transformers/file_utils.py	68.49% <0.00%> (-0.37%)	⬇️
src/transformers/modeling_utils.py	90.94% <0.00%> (-0.13%)	⬇️

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[julien-c]

LGTM

[monologg]

@LysandreJik @VictorSanh

Hi:) Can you please check this PR? This one makes issue on Korean BERT. (which use pad_token_id=1 and unk_token_id=0)

I hope this PR will be applied on the next version of transformers library:)

[VictorSanh]

lgtm!

[LysandreJik]

LGTM

[monologg]

@julien-c

Can you merge this PR? Thank you so much:)