Exclude Special Tokens from Masking in mlm_masking

[jihobak]

Modified the mlm_masking method to exclude special tokens from the masking process.
Previously, the method applied masking uniformly to all tokens without considering special tokens.

Changes

• Added a special_tokens parameter to allow specifying token IDs (e.g., [CLS], [SEP]) to be excluded from masking.
• Updated the eligible mask logic to filter out both special tokens and pad tokens before applying masking.
• Refactored the masking process to ensure that only eligible tokens are considered for the 80/10/10 masking scheme.

Discussions

• Issue #212 regarding this change.
• If needed, I will add unit tests for this function in a subsequent commit.

Tests

• Is the new feature tested? (Not always necessary for all changes -- just adding to the checklist to keep track)
• Have you ran all the tests?
• Do the tests all pass?
• If not, have you included an explanation of which tests this PR breaks and/or why (below this checklisT)