Fix bug in memory planning (#1300)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1300

This diff fixes bug in memory planning where const tensors are also
memory planned for.

A proper fix should be a done a bit differently as mentioned int he
comment.

Reviewed By: ydwu4

Differential Revision: D51625496

fbshipit-source-id: 746963beea027558f6eafc6d799ce7f9d4770933