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Summary:
Pull Request resolved: https://github.com/facebookresearch/d2go/pull/419

This diff adds a metadata class `LayerLossMetadata` to help keep track of the losses we want to compute over layers. The class contains the type of loss, loss name, and layer names.

This diff adds a helper function to iterate over a list of `LayerLossMetadata` and return a dict containing the results.

Reviewed By: chihyaoma

Differential Revision: D40286564

fbshipit-source-id: b269dc63cc90a437ca279379d759c3106016327c