Adds functionality for Slice Node in sytorch#185

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drunkenlegend wants to merge 3 commits intomasterfrom slice

+420-31

Commits on Jul 10, 2023

initial implementation for slice correctness verified
drunkenlegend
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Commits on Jul 13, 2023

made the slice node general, now supports vectors in axes, and optional values
drunkenlegend
committed
extended functionality for -ve axes and steps
drunkenlegend
committed