Add an API to get external value shape

[digantdesai]

• Add a way to get the shape of the external value. Typically used to get dynamic shape of an external output value after runtime invocation.

• Add a couple of tests,

  • Add: standard tests w/o any reshape. Also tests invalid cases.
  • SDPA: check shapes for smaller and larger outputs than subgraph creation

[digantdesai]

cc @alankelly

[digantdesai]

@fbarchard - can we merge this?

[fbarchard]

The review is approved but requires someone with write access to merge/land the PR

[alankelly]

Our bot which handles inporting/exporting PRs to github was temporarily broken and I think this PR got mixed up somehow with this.

I have added created a new PR with the modified comment so will close this.