Fix type notation of merges in BPE Python binding

[Coqueue]

Heya, this change is to correct type notations of merges in Python binding of BPE as the counterpart in Rust implementation expects a file name string or Vec<(String, String)> instead of a mapping from a tuple of integers to another.

[Narsil]

Why do you think the annotations are incorrect ? The 2 things are unlinked and the signature here is correct.

[Coqueue]

Hi Nicolas, thanks for taking a look.

The 2 things are unlinked and the signature here is correct.

I wonder if this is true.
Taking the Python SentencePieceBPETokenizer class as an example, the merges annotated as Optional[Union[str, Dict[Tuple[int, int], Tuple[int, int]]]] is only used to initialize the BPE class:

if vocab is not None and merges is not None:
        tokenizer = Tokenizer(BPE(vocab, merges, dropout=dropout, unk_token=unk_token, fuse_unk=fuse_unk))

where the BPE class is generated from the Rust implementation mentioned in the original post.

BPE class also contains below comment:

class BPE(Model):
    """
    An implementation of the BPE (Byte-Pair Encoding) algorithm

    Args:
        vocab (:obj:`Dict[str, int]`, `optional`):
            A dictionary of string keys and their ids :obj:`{"am": 0,...}`

        merges (:obj:`List[Tuple[str, str]]`, `optional`):
            A list of pairs of tokens (:obj:`Tuple[str, str]`) :obj:`[("a", "b"),...]`

At least this comment gives a different type annotation for merges.

Why do you think the annotations are incorrect?

I encountered an issue initializing SentencePieceBPETokenizer with the given type annotation, but succeeded following the type annotation given in the comment of BPE class.

The type annotation of merges being Dict[Tuple[int, int], Tuple[int, int]]]] should actually be for MergesMap in Rust, as an alias of HashMap<Pair, (u32, u32)>, which is built internally in Rust from merges.

As you closed the PR as a repo collaborator, I'll provide the above context -- I'll be more than happy to learn more if I was wrong.