Decide on inclusion of executable graph update feature

[EwanC]

Being able to update an executable graph with new inputs is a feature that is available in the Codeplay vendor graphs extension, but not the original oneAPI extension.

Such an extension requires support in the underlying API to be performant, e.g cl_khr_command_buffer_mutable_dispatch.

We should decide if this feature is a requirement for making the ML applications we are interested in performant, or if it can be removed from SYCL_EXT_ONEAPI_GRAPH

[EwanC]

I spotted that GROMACS CUDA graph backend found this feature useful - see https://hihat.opencommons.org/images/d/d1/GromacsGraphsHihatJan21_v2.pdf slide 11

[EwanC]

Closing this issue as the equivalent CUDA functionality was found in pytorch, tensorflow, and gromacs. Removed open issue from spec in #108