Special tokens masking candidates

Based on the masking implementation in the `transformers` library, special tokens (e.g., [CLS], [SEP]) should be excluded from the masking process. However, upon reviewing the implementation in sequence_packer.py,
https://github.com/AnswerDotAI/ModernBERT/blob/8c57a0f01c12c4953ead53d398a36f81a4ba9e38/src/sequence_packer.py#L284
it appears that these tokens are currently being treated as valid masking candidates.

Could you please confirm if this behavior is intentional? If not, I suggest updating the masking logic to explicitly exclude special tokens. For instance, adding a condition to filter out these tokens before applying the mask would ensure consistency with the `transformers` library's approach. Additionally, incorporating unit tests to verify that special tokens remain unmasked would improve code reliability.

Am I correct in my understanding, or is there something I might be missing?

Thank you for looking into this.

Start at src/sequence_packer.py around line 284 and compare candidate selection with the masking behavior in the transformers library. Confirm whether [CLS] and [SEP] are included, then add unit coverage showing special tokens remain unmasked if the behavior is unintended.

Written by the indexing model from the issue text.