Fix variable number of inputs in mps_example.py (#918)

Summary:
- fix models with variable number of inputs (>1). Needed for the models `add`, `linear`, `add_mul` where the inputs array is usually made out of 2/3 tensors.
- llama2 is also working correctly through MPS delegate. Set the `atol` to 1e-4 when doing the bundled program comparison check with the eager output from CPU.

Pull Request resolved: https://github.com/pytorch/executorch/pull/918

Reviewed By: kirklandsign

Differential Revision: D50301445

Pulled By: kimishpatel

fbshipit-source-id: 38576363de60fc45dabc92833723cf8f293cc7c5