Functions to create external objects from arrayfire

First step would be to create these objects to be copies of arrayfire arrays
- [ ] pycuda
- [ ] pyopencl
- [ ] numba
- [ ] pytorch ?

The issue names no files, tests, or entry points, so first clarify the supported external object types and the intended copy behavior for each listed target. Define completion as documented functions that create copies for the agreed targets, with tests showing the resulting objects and their data match the source arrays.

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