Sgm analytic moments

[duncan-clark]

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x ] 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

Implemented analytic moment accountant see : https://github.com/pytorch/opacus/issues/379 https://arxiv.org/abs/1808.00087

How Has This Been Tested (if it applies)

Recreated the CIFAR10 Opacus tutorial contained in the opacus docs here

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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