[Draft][XNNPACK] Share workspace across delegate instances

[digantdesai]

This require us to move to create_runtime API v2 -> v4. This should be backwards compatible (i.e. old PTE should be able to load), and should also be supported on slightly older version of XNNPACK 3p library given the v4 got introduced 2 years ago.

This patch add a new workspace pointer member in the XnnpackBackend instance.

This should be done also for the weight cache. Which is left as a TODO here for now.

[pytorch-bot[bot]]

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As of commit 67c931adea8f007886ef16b0415682252d1fa4ff with merge base ba3448c3ce37e01de84abe07a35eb87cf5b2ae45 (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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[kirklandsign]

I will give a bit more patience for bot to merge this