initialize/set backend in runKernelFunc

[tafsiri]

Fixes #4480

Initialize backend in runKernelFunc if its not already initialized/set.

It manifests in the bug above because linspace no longer calls other ops which would initialize the backend (usually by creating some tensor). It also affects op like range which call runKernel before doing any tensor creation or anything else. The bug will only appear if linspace or range is the first op called, and no any other action that would cause the lazy setting of a backend to have completed (like tensor creation).

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[annxingyuan]

Interesting! Thanks Yannick. Could you add a bit more context to the comment - e.g. mention that certain ops don't cause backend initialization anymore?

[tafsiri]

@annxingyuan Thanks. I've updated the comment to mention lazy backend initialization and how that interacts with ops/kernels broadly, I didn't want the comment to be too tied to the ops mentioned in this PR description to keep it as relevant as possible 6 months+ from now, so I tried to find a general way to express it.