[hard] add masked.median

[ManfeiBai]

pytest test failed with error: RuntimeError: Object of type 'XLATensor2' is not an instance of 'sequence' at current branch

[ManfeiBai]

masked.median is decomposed with nanmedian.dim, and nanmedian.dim will return two result according to https://pytorch.org/docs/stable/generated/torch.nanmedian.html, but my logic only return one value,

so the logic of nanmedian.dim need a sequence of tensor return, when I only return one tensor, it shows error RuntimeError: Object of type 'XLATensor2' is not an instance of 'sequence'