feat: Add HuggingFace-style skip_special_tokens and batch_decode support (#403)

[lakshitsoni26]

• Fixes Improve tokenizer decode #403 by adding batch_decode, skip_special_tokens, and clean_up_tokenization_spaces to the tokenizer API.
• Implemented a clean_up_tokenization helper to exactly mimic HuggingFace's internal string-cleaning logic (stripping spurious spaces before punctuation).
• Added batch_decode and the new decoding arguments to SimpleTokenizer and TikTokenTokenizer.
• Added pass-through wrappers for the new arguments in HFTokenizer and SigLipTokenizer.
• Bug Fix: Fixed an edge case where padding tokens could accidentally bleed into the output string in SimpleTokenizer.
• Testing: Added comprehensive unit tests in tests/test_tokenizer_decode.py covering all tokenizer variants (100% test coverage for the new features).
• Zero Regressions: Preserves exact backwards compatibility by defaulting the new boolean arguments to False across all signatures.

[rwightman]

@lakshitsoni26 so there are some risks here, and decisions to be made, it's not a 'zero regression' case... there's also some shifting ground re other significant additions (e.g. #2713) so am going to defer tokenizer cleanup until a bit later

[lakshitsoni26]

Understood! Thanks for taking a look. I'll leave this PR open for now — just ping me whenever the dust settles on #2713 and you are ready to revisit the tokenizer cleanup. Happy to update this PR to resolve any conflicts then!