export_model tweak

[zphang]

Convert export_model to export paths:

• model/{model_type}.p --> model/model.p
• model/{model_type}.json --> model/config.json

Additionally:

• Replace model_type with hf_pretrained_model_name_or_path to exported summary config.json

[codecov]

Codecov Report

Merging #1277 (49be2c8) into master (84f2f5a) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1277      +/-   ##
==========================================
- Coverage   48.47%   48.47%   -0.01%     
==========================================
  Files         163      163              
  Lines       11214    11213       -1     
==========================================
- Hits         5436     5435       -1     
  Misses       5778     5778              

Impacted Files	Coverage Δ
jiant/proj/main/export_model.py	89.28% <100.00%> (-0.37%)	⬇️

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