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Base class for resolver.
Resolver is the logical unit that will be used optionally for input selection.
A resolver subclass must override the resolve() function which takes a
read-only MLMD handler and a dict of
resolve( metadata_handler, source_channels )
Resolves artifacts from channels by querying MLMD.
metadata_handler: a read-only handler to query MLMD.
source_channels: a key -> channel dict which contains the info of the source channels.
a ResolveResult instance.