Implement operator<<() for EValue (#479)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/479

Add a helper utility to print `EValue`s. Doesn't format multidimensional data like PyTorch does, but we can improve that in the future if we want to.
ghstack-source-id: 201868939
exported-using-ghexport

Reviewed By: JacobSzwejbka

Differential Revision: D49574853

fbshipit-source-id: c8757790fb260785cd8a8c39e1f691d097f9e5b5