contrastive loss batch size

Figured out a solution. In this line, the argument parsed into the definition of contrastive loss have 'batch_size=batch_size*2'. However, in the function, the batch size has been multiplied again. The problem was fixed when I set 'batch_size=batch_size' in the definition. If it is the case, I would send a PR to fix that.

Originally posted by @Eason-Qin in https://github.com/Project-MONAI/tutorials/discussions/624#discussioncomment-3389489

this looks like a bug, @finalelement could you please help confirm?

addressed by https://github.com/Project-MONAI/MONAI/pull/5093