Update session_mixin.py

[imrankh46]

replace tokenizer by processor

if processor is not None:
    processor.save_pretrained(output_dir)```

SUMMARY:
"please provide a brief summary"


TEST PLAN:
"please outline how the changes were tested"

[kylesayrs]

Hi @imrankh46, this looks good to me! However, I cannot merge this until

1. This change is moved to the branch on Update text_generation.py #938
2. The commented code is removed
3. The commits are signed for the DCO

[kylesayrs]

Handled by #955