Set image dim=2 for the computation of image embeddings

[caroteu]

@anwai98 The image dimension needs to be passed here, else datasets with (1,width,height) type images will cause an error.

[anwai98]

Hi @caroteu,
We shouldn't fix the number of dimensions in the class method itself.
Also, if we have monochannel inputs, we should use the inputs as (H x W) (which is the expectation in micro-sam)