[RudrenduPaul]

What this PR does

Adds missing return type hints to three methods in
bindings/python/py_src/tokenizers/implementations/byte_level_bpe.py:

• from_file(vocab_filename: str, merges_filename: str, **kwargs) -> "ByteLevelBPETokenizer"
• train(...) -> None
• train_from_iterator(...) -> None

This follows the same typing-consistency pattern as #2211, which added
the equivalent missing return hints to BaseTokenizer.save,
save_model, and to_str. ByteLevelBPETokenizer (a BaseTokenizer
subclass) has the identical gap: every argument was already annotated,
but the return type was never added to these three methods.

Why these types are correct

• from_file is a @staticmethod that returns
  ByteLevelBPETokenizer(vocab, merges, **kwargs), i.e. a new instance
  of the class — hence -> "ByteLevelBPETokenizer", using the quoted
  self-referential forward-reference convention (the file has no
  from __future__ import annotations, and the class name is not yet
  bound in scope while its own body is being defined). This mirrors the
  identical convention already used on the sibling
  CharBPETokenizer.from_file -> "CharBPETokenizer" pattern for the same
  situation elsewhere in implementations/.
• train and train_from_iterator both call self._tokenizer.train(...) /
  self._tokenizer.train_from_iterator(...) without a return statement,
  so they implicitly return None — hence -> None.

Scope note

Ran gh pr list --repo huggingface/tokenizers --search "byte_level_bpe" and
--search "ByteLevelBPETokenizer" before starting; no open or closed PR
currently touches this file. Re-checked immediately before opening this PR.

No behaviour change

Type-hint-only change on method signatures. No logic, docstrings, or other
files were modified.

Testing

• ruff check bindings/python/py_src/tokenizers/implementations/byte_level_bpe.py — all checks passed.
• ruff format --check bindings/python/py_src/tokenizers/implementations/byte_level_bpe.py — passes, file already formatted.
• python3 -c "import ast; ast.parse(...)" on the modified file — passes.

Note: Claude Code was used to assist in drafting this change. All changes were reviewed by the submitter.