Allow SuperPoint_Open weights to be loaded from local cache

[aduverger]

Hello,
This PR addresses an issue we encounter when having to use SuperPoint in a production environment.
Currently, the pre-trained model weights MUST be downloaded from https://github.com/rpautrat/SuperPoint/raw/master/weights/superpoint_v6_from_tf.pth.

This is not compatible with a production env:

• You don't necessarily want your model to access internet
• GitHub could be down for a moment, thus your endpoint could be down as well
• Rémi Pautrat could rename / delete that file, just to watch the world burn 😄

This PR allows the users to specify a weights parameter in the config input (similar to LightGlue), so that the weights are loaded from cache instead of from rpautrat's repo.

[aduverger]

Anyone could have a look at this PR please :) ?
@sarlinpe, @Phil26AT, @rpautrat

[sarlinpe]

Nice, thank you! And apologies for the late reply.