3/N: CPUOffloadedRecMetricModule (#3428)

Summary:
Pull Request resolved: #3428

This diff adds the basic building blocks for a zero overhead RecMetrics implementation. Follow up patches will contain integration with users of torchrec.

One of the main pain points of using RecMetricModule is that metric updates and computes are done synchronously. In training jobs, there has been cases where metric updates take +20% of a training iteration. Metric computations, although less frequent, can takes over a couple of seconds.

CPUOffloadedRecMetricModule aims to perform all metric updates/computes asynchronously, completely removing them from the critical path.

This patch adds:
- CPUOffloadedRecMetricModule: RecMetricModule that offloads metric update() and compute() to CPU using background threads and dual queues.

Reviewed By: iamzainhuda

Differential Revision: D83773529

fbshipit-source-id: bfe4517b51fd693f9a891d13075663ec123b6ff4

Author: jeffkbkim