support emit sym value from delegate (#3103)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3103

For dynamic shape, if delegate output is dynamic shape, the return might be something like `(s0, x, y)`, and `s0` is a sym type while others are fake tensor. In this case, we will emit the sym value (including `SymFloat`, `SymBool`, `SymInt`) to a unique Evalue.

Since the sym type node will have an empty spec, we use the `node.meta['val']` to find out it's a sym type node.

Reviewed By: mcr229

Differential Revision: D56176100

fbshipit-source-id: a4ddc7225ed014c59ceb9fa8ba4a9cb394af00e5