Transformer chaos containment

[texasmichelle]

Before more transformer-based language models are added to this repo, let's pay down some debt on the ones we have:

• Improve BERT model #315 Various BERT cleanup
• [BERT-CoLA] Add epochs #480 Use Epochs as a data loader in CoLA
• [BERT-CoLA] Data source resolution #488 Data source resolution in CoLA
• [BERT-CoLA]: Use common file extraction functions #489 Use common file extraction functions in CoLA
• Disentangling CoLA and BERT #432 Disentangle CoLA and BERT
• Verify BERT variants #433 Verify BERT variants (ALBERT and RoBERTA are untested)
• GPT2 and BERT alignment #434 GPT-2 and BERT alignment

This will make additional models easier to add and maintain.