def pop_max_vocab_size(params: Params) -> Union[int, Dict[str, int]]:
    """
    max_vocab_size limits the size of the vocabulary, not including the @@UNKNOWN@@ token.

    max_vocab_size is allowed to be either an int or a Dict[str, int] (or nothing).
    But it could also be a string representing an int (in the case of environment variable
    substitution). So we need some complex logic to handle it.
    """
    size = params.pop("max_vocab_size", None)

    if isinstance(size, Params):
        # This is the Dict[str, int] case.
        return size.as_dict()
    elif size is not None:
        # This is the int / str case.
        return int(size)
    else:
        return None
max_vocab_size limits the size of the vocabulary, not including the @@UNKNOWN@@ token.

    max_vocab_size is allowed to be either an int or a Dict[str, int] (or nothing).
    But it could also be a string representing an int (in the case of environment variable
    substitution). So we need some complex logic to handle it.