Assign trivially moveable values to const-ref variables (#2834)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2834

The [`toType()` methods return a reference](https://github.com/pytorch/executorch/blob/be618c27ee75fb949f3479115f4ea585ea291451/backends/vulkan/runtime/graph/containers/Value.h#L222) so there's no need to copy the return value.
ghstack-source-id: 221721753
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Reviewed By: copyrightly

Differential Revision: D55701570

fbshipit-source-id: 14fb5089129c89196216403ff317f21e431e3fd2