add code for continuing pre-training from an ELECTRA checkpoint

[tuvuumass]

• add a variable init_checkpoint to configure_pretraining.py
• add code for continuing pre-training from an ELECTRA checkpoint to run_pretraining.py
• update README (instructions for continuing pre-training from an ELECTRA checkpoint + remove unnecessary stuff, i.e., "num_train_steps": 4010000 or "learning_rate": 2e-4 * (4e6 + 10000) / 10000, as we can specifify the actual "num_train_steps" (i.e., 10000 in this case) and learning rate (2e-4) now.

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[tuvuumass]

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