Default nanos in DurationStruct to zero
Default nanos in DurationStruct to zero

Summary: The main reason for this is so that we can specify minutes, hour durations without the need to add "nanos":0 every time.

Reviewed By: Alfus

Differential Revision: D37956267

fbshipit-source-id: 1719b7a79e0a4b135619d4eab30ebe30b546b2aa

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