Fix: add missing SAFE_WEIGHTS_INDEX_NAME to __all__ in constants.py

[Pranavi125]

This PR adds the missing SAFE_WEIGHTS_INDEX_NAME constant to the all list in constants.py.

Why:
Without this, the constant isn’t exported when using
from transformers.utils.constants import *.

Impact:

• Keeps the constants module consistent.
• No functional changes or breaking impact.

[vasqu]

These constants are used internally only, so I'm not sure why we'd want to include them here

Is there any issue or example you have? SAFE_WEIGHTS_INDEX_NAME doesn't even exist in the file

[vasqu]

These changes here are unrelated, no?