Update loss.py

[johnson-magic]

fix bugs when pos_inds.numel()=0, reg_targets_flatten and anchors_flatten is empty tensor. It will be wrong for some tensor operator in self.compute_centerness_targets function. For example the tensor operator "left_right.min(dim=-1)" the error info is " RuntimeError: cannot perform reduction function min on tensor with no elements because the operation does not have an identity"