Make dynamic shape based export selectable (#4100)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4100

Most backends like mps, qnn, coreml dont support dynamic shape. Particularly
for mps, query to .item(), data dependent dynamic shape, results in lowering
error. This error is likely fixable in the delegate however not clear if the
delegate is meant to support dynamic shape or not.

//pre-existing lint error
bypass-github-export-checks

Thus making dynamic shape specific change selectable for now.
ghstack-source-id: 232298483

Reviewed By: cccclai

Differential Revision: D59236591

fbshipit-source-id: 2870148373f2f9f0ee9d4646355ac0664d2c2f68