Added TryUnstack for tensors.

[swfsql]

• Adds the try_/unstack method for tensors.
• Also added from_fn for Arrays.
• This is similar to Adds unstack #831 by @emchristiansen - thanks! I used your code for reference - but the difference is that this one also allows for unstacking into arrays instead of only unstacking into Vecs, which is closer to how the interface for stack currently works.
  • But note that this is only useful for "shape certainty" purposes, there shouldn't be much difference in performance.
  • Also, in this implementation, a two-pass array/vec construction is necessary, which does have a performance hit even for the Vec case.
    • This should be avoidable by having a method allowing to merge a tape into a tensor that already had a OwnedTape (instead of moving the tensor - which is necessary since the type is going to change - and splitting it), but in order to avoid adding such functionality I opted for a two-pass array/vec construction.
• Together with Add split_tensor_along method #917, may close Please add a split method, to iterate over subtensors of the given tensor #825.

Remaining Tasks

• Decide on how to manage non-contiguous data - it's mandatory. Possibly panic at runtime.
• Add Cuda kernel.
• Consider adding function to merge a OwnedTape into a tensor which already has a OwnedTape, with a &mut to the tensor.
• Consider adding a layer, even though TryStack doesn't have one yet.
• Consider adding a _vec method that always realizes the first dimension into a usize before unstacking.
• Consider allowing the unstack for an arbitrary axis, similar to the concat interface.
  • Note: This is not present on the stack interface; Maybe this can be possible with a helper function that always transmutes that axis as the first one and then calls the unstack; Also this could be made by a future pr.