fix: Add pad_token to special_tokens_dict when pad_token == eos_token

[Abhishek-TAMU]

Description of the change

Add pad_token to special_tokens_dict when pad_token == eos_token to increase F1 micro score for Granite3.0-8b model.

Related issue number

Issue:#343
Issue 343 modifies pad_token of tokenizer but in this PR adds it to special_tokens_dict for model resizing.

Reason of this case of pad_token == eos_token:
When tokenizer has PAD token equal to EOS token, then the DataCollatorForCompletionOnlyLM masks the EOS token from labels of dataset batch and hence its not used in loss calculation and hence it leads to repetitive predicted_target during inference time.

How to verify the PR

Was the PR tested

• I have added >=1 unit test(s) for every new method I have added.
• I have ensured all unit tests pass

[github-actions]

Thanks for making a pull request! 😃
One of the maintainers will review and advise on the next steps.

[Ssukriti]

thank you!

for context for others: Without this fix the token values are not actually getting modified, to set a unique pad token and then EOS is masked