All right, changing absolutely nothing except model_type, I no longer get “IndexError: index out of range in self”, and it seems to be training? But yet, it still reads in and displays this from config.json:
[INFO|configuration_utils.py:553] 2021-06-07 09:17:42,045 >> Model config RobertaConfig {
  "architectures": [
    "RobertaForMaskedLM"
  ],
  "attention_probs_dropout_prob": 0.1,
  "bos_token_id": 0,
  "eos_token_id": 2,
  "gradient_checkpointing": false,
  "hidden_act": "gelu",
  "hidden_dropout_prob": 0.1,
  "hidden_size": 768,
  "initializer_range": 0.02,
  "intermediate_size": 3072,
  "layer_norm_eps": 1e-05,
  "max_position_embeddings": 207,
  "model_type": "roberta",
  "num_attention_heads": 6,
  "num_hidden_layers": 3,
  "pad_token_id": 1,
  "position_embedding_type": "absolute",
  "transformers_version": "4.7.0.dev0",
  "type_vocab_size": 1,
  "use_cache": true,
  "vocab_size": 52000
}

Updated command, only changed model_type:
python run_mlm.py \
    --model_type bert \
    --tokenizer_name /home/cleong/projects/personal/colin-summer-2021/EsperBERTo/ \
    --train_file /home/cleong/projects/personal/colin-summer-2021/data/oscar.eo.txt \
    --max_seq_length 512 \
    --do_train \
    --output_dir ./output/test-mlm