:bug: [BUG]: Incorrect get_num_losses return value when using custom loss function

[zinccat]

The implementation of get_num_losses in domain.py is summing up the number of output names in constraints. Yet this is not the case when using custom loss function, e.g. when using variational constraint like the case in https://docs.nvidia.com/deeplearning/modulus/modulus-sym/user_guide/intermediate/variational_example.html