TokensLoader with parallel meta field

[Aceticia]

🚀 Feature

Add a TokensLoaderWithMeta class that stores some additional parallel meta data with the tokens. It can be used to store data with a bit more structure than flat sequence, like image tokens. Here's an example:

{
    "token":     [1,2,3,4,5],
    "token_x": [0,0,0,1,1],
    "token_y": [0,1,2,0,1]
}

Notice they all have the same length.

Motivation

I've been using TokensLoader to train models and find it to be really handy. But it's unfortunately a bit difficult to use when I want to experiment with different positional encoding schemes.

Alternatives

The alternative is to create a normal LitDataset with these. But it is less efficient to store, load, and harder to pack samples.