Make loss debgging easier

[josephdviviano]

https://github.com/GFNOrg/torchgfn/blob/d9ca55815c43e129fad95fdcb408280c90b19963/src/gfn/gflownet/flow_matching.py#L98

There should be a debug flag you can use to inspect the logits going into the loss and other issues like this