Add support for .eval()

[netw0rkf10w]

Fixes #758 and #719

These two lines of code are sufficient to enable many extrapolation algorithms, including the weight averaging variants such as stochastic weight averaging, exponential moving average, schedule-free (cc @adefazio).

[github-actions]

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[priyakasimbeg]

Closing this in favor for solution implemented in #789 as discussed in the working group meeting.