caffe2 - Util to cleanup external inputs and outputs from a NetDef (#18194)

caffe2 - Util to cleanup external inputs and outputs from a NetDef (#18194)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18194

Add a util method to cleanup external inputs and outputs from a NetDef

The following conditions will be met after the modification
- No duplicate external inputs
- No duplicate external outputs
- Going through list of ops in order, all op inputs must be outputs
from other ops, or registered as external inputs.
- All external outputs must be outputs of some operators.

Reviewed By: ZolotukhinM

Differential Revision: D14528589

fbshipit-source-id: c8d82fda1946aa3696abcbec869a4a8bb22f09b6