Extend support for scalars and scalar lists in Value class (#2271)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2271

## Context

This changeset enables serialization and execution of Operators with arbitrary function signatures. Previously, only operators with a very specific schema were supported (2 inputs, 1 output).

This is achieved by extending the `Value` class (which is essentially a tagged union) to support all necessary types. All objects needed to execute an operator are now serialized/deserialized as a tagged union.

This changeset also refactors `VulkanBackend.cpp` by introducing `GraphBuilder` which makes constructing a `ComputeGraph` from a serialized flatbuffer much clearer.

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Reviewed By: jorgep31415

Differential Revision: D54561567

fbshipit-source-id: c66d5d6b49e33a4373c7d6e83d510bc85da4b5d4