Correct GPT-J voab_size
# What does this PR do?

Corrects GPT-J `vocab_size`. GPT-J has 50400 `vocab_size` but the tokenizer’s vocab size is 50257. And the `run_clm.py` script always resizes the token embeddings using `len(tokenizer)`, so when the model is fine-tuned with that script, it changes the embedding size, which results in shape mismatch as described in #13499.
These extra tokens are for the sake of efficiency on TPU and are not used by the model.

This however would break all existing downloaded models, since those checkpoints will have 50400 `vocab_size`. The solution would be to manually change the `vocab_size` in the `config.json` file to 50257.


Fixes  #13499, Fixes #13581
Should we update the official GPT-J config then as well? <|||||>Yes, will update the official configs and weights as well, if the PR is approved.<|||||>T5 actually has the same issue where there is a mismatch between model's vocab size and tokenizer's vocab size which led to quite some confusion so very much in favor of changing it here (especially since there hasn't been an official release yet).

Note that the official GPT-J repo has two branches so we should make sure to update both<|||||>Yes, the extra tokens are there for efficiency reasons on TPU, since the original implementation uses model parallelism.
I agree that we could modify the script and maybe add a flag, so embeddings are only resized if the user explicitly passes the flag.

But if a user wants to add new tokens to the tokenizer and resize embeddings, it will reduce the `vocab_size`, which is a bit confusing IMO (that is the case with T5)<|||||>I understand the change and think that this will indeed lead to painful errors. If modifying the model size is not an option for optimization's sake and not resizing the model leads to painful errors, how about resizing the tokenizer by adding new (unused) tokens to it?

Resize the tokenizer to 50400 so that it matches the model's vocab size, and put unused values in the unused range. WDYT?<|||||>I think @LysandreJik 's solution is the best!<|||||>Think I'm fine with adding unused tokens to the tokenizer. 

I'm a bit worried about the community being confused as GPT-J uses the official GPT2 tokenzier which has 50257 tokens (with the last token being the EOS token). So reading that GPT-J uses the official GPT2 tokenizer: https://github.com/kingoflolz/mesh-transformer-jax#model-details with 50257 and then seeing a different vocab size in the hub might be a bit confusing for people (maybe with a good warning it's fine?)

Also think though that @LysandreJik is the best solution here<|||||>Good call! Maybe a mention on the model card mentioning why the model has a vocab size of 50400 instead of the 50257 tokens it has been trained with is a solution? <|||||>cc @StellaAthena <|||||>Sounds good to me @LysandreJik! 

> Maybe a mention on the model card mentioning why the model has a vocab size of 50400 instead of the 50257 tokens it has been trained with is a solution?

I think the model card already mentions this and maybe we could also put this in docs as well.<|||||>[added extra tokens](https://huggingface.co/EleutherAI/gpt-j-6B/commit/d3109d20aec493086dfca34b79bc233c48419949) and updated the tokenizer, #13696 adds a note about this in the docs.
Will close this PR now.