Sum on 3 dimensions raises NotImplementedError

[CharlelieLrt]

This might be already documented elsewhere, but when trying to execute a numpy-based code with cunumeric in place of numpy, some of the reductions such arr.sum(axis=(-3, -2, -1)) fails with NotImplementedError: Need support for reducing multiple dimensions.

A simple workaround is to instead do it dimensions by dimension with arr.sum(axis=-1).sum(axis=-1).sum(axis=-1), but it would be nice to have the multi-dimensional reduction. Having this limitation mentioned in the documentation of cunumeric.sum could also be useful.