[FEATURE] Add metod to Criteria to check whether the model was downloaded or not

[InAnYan]

Description

Criteria.loadModel method won't tell whether:

1. The model wasn't downloaded, and it will be downloaded right away.
2. The model was already downloaded, so no Internet connection will be created, and DJL will reuse existing files.

For the first case in the real app, it would be useful for users to make a background task saying "Downloading model...", however for the second case no need to tell users about anything.

But because with Criteria.loadModel we can't distinguish anything, the user is constantly prompted "Downloading model...", while it's possible that it was already downloaded.

I propose to add a method like Criteria.checkIfModelWasDownloaded, to distinguish these two cases.

Will this change the current api? How?

This feature probably will introduce only one new method, so no breaking changes 😄

Who will benefit from this enhancement?

Users of you library, who develop applications for non-computer-science-background users

References

[InAnYan]

@frankfliu Thank you! Will try to use it as soon as possible!