Here’s what I used to add some tokens:
from transformers import T5Tokenizer
from transformers import T5ForConditionalGeneration

tokenizer = T5Tokenizer.from_pretrained('t5-small')
model = T5ForConditionalGeneration.from_pretrained('t5-small')

local_dir = "./cryptic_special"
model_name = "t5-small"

special_tokens = ["<DEFN>",
                  "<ANAG>",
                  "<ANS>",
                  "<INDIC>"]

tokenizer_special = T5Tokenizer.from_pretrained(model_name, additional_special_tokens=special_tokens)
model.resize_token_embeddings(len(tokenizer_special))
tokenizer_special.save_pretrained(local_dir)
model.save_pretrained(local_dir)

Then you just adapt the fine_tune script to point to the local_dir (for model and tokenizer)