Compiler error and some DNNROI related updates

[HaiwangYu]

No description provided.

[HaiwangYu]

tested OK:
https://indico.bnl.gov/event/25041/

[brettviren]

Hi @HaiwangYu

I think this PR is okay but the lack of torch::Tensor::index() in v1.6 surprises me. Is there really no way to index tensors in torch 1.6? I know one can construct a strongly typed and dimensioned "accessor", at least in LibTorch 2.x, but this lacks the "pythonic" indexing syntax.

https://pytorch.org/cppdocs/notes/tensor_basics.html#efficient-access-to-tensor-elements

[HaiwangYu]

There could be alternatives like index_select? Haven't tried that yet.

[wenqiang-gu]

I reviewed the code in OmnibusSigProc, and everything looks good to me.