Feng Liu 3d7ab2d265 Add support to opaque elements attributes
For some of the constant vector / tesor, if the compiler doesn't need to
interpret their elements content, they can be stored in this class to save the
serialize / deserialize cost.

syntax:

`opaque<` tensor-type `,` opaque-string `>`

opaque-string ::= `0x` [0-9a-fA-F]*
PiperOrigin-RevId: 218399426
2019-03-29 13:36:45 -07:00
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