[ET-VK] Store unique ptr to Tensor in Value instead of inlined tensor object, to reduce Value struct size from 448 to 80 bytes.

Pull Request resolved: https://github.com/pytorch/executorch/pull/7145

This diff aims to reduce the size of the Value struct in the Executorch Vulkan runtime by storing a unique pointer to the Tensor object instead of an inlined tensor object. This change reduces the size of the Value struct from 448 bytes to 80 bytes, which can improve performance and reduce memory usage.
ghstack-source-id: 256911524
@exported-using-ghexport

Differential Revision: [D66655991](https://our.internmc.facebook.com/intern/diff/D66655991/)

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Co-authored-by: Vivek Trivedi <5340687+trivedivivek@users.noreply.github.com>