[RFC][Relay][Topi] Hashtable support

I encountered similar issue with HashTable for TensorFlow :

NotImplementedError: The following operators are not implemented: {'DivNoNan', 'LookupTableImportV2', 'HashTableV2', 'AsString', 'StringToHashBucketFast', 'LookupTableFindV2', 'LookupTableSizeV2'}

and then noticed this RFC discussion thread. @lfengad @tqchen wondering if we have any good solution for LookupTable? I checked several of my models, seems these are very common among all my models, mostly introduced by embedding/TensorFlow-Transform.