Enable customization of party performing public computations

[lvdmaaten]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Docs change / refactoring / dependency upgrade

Motivation and Context / Related issue

The current implementation of CrypTen's MPC protocol assumes that there is always a party with rank 0. This party is hardcoded to be responsible for performing public addition, public XOR, NOT, and for requesting Beaver triples. In setups in which party 0 exists but does not partake in the computations (that is, is not in DistributedCommunicator.main_group) incorrect results will be produced..

This PR introduces class variables that allow a user to customize which party is in charge of those things.

How Has This Been Tested (if it applies)

Unit tests for public addition, public XOR, and NOT included. There are no unit tests for TTP and TFP, but I have tested the changes locally.

Checklist

• The documentation is up-to-date with the changes I made.
• I have read the CONTRIBUTING document and completed the CLA (see CONTRIBUTING).
• All tests passed, and additional code has been covered with new tests.

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