multiscale segmentation loss API

[wyli]

Is your feature request related to a problem? Please describe.
this ticket looks for a generic wrapper API to compute a given segmentation loss at multiple visual scales. the API should be compatible with the existing single-scale segmentation loss in monai

see also:
deepreg example: https://github.com/DeepRegNet/DeepReg/blob/a182f9d86fbd1ff9e392c511ff89403971be6454/deepreg/loss/label.py#L42-L47

[kate-sann5100]

If no one is already implementing this, I would be happy to implement. @wyli @Nic-Ma