Add rudimentary NestedTensor.sum(dim) (#82387)
A first step towards adding dimension-wise reductions to NestedTensor,
- Assumes tensors in the nested tensor as well as the buffer of the nested tensor are contiguous
- Always enforces `keepdim=True`
- Only supports reduction across the last dimension
- No support for acctype (`dtype` argument)
- No autograd support
- CPU only
Next steps would be to add support for the above. For now this basic support is for prototyping to make sure `NestedTensor` can be used as an API for segment reductions.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82387
Approved by: https://github.com/jbschlosser
