[MINOR] Parsing IR fix.

[aristotelis96]

While parsing a matrix object the "sparsity" and "rerpesentation" arguments can both exist. In that case trying to parse one of them while multiple exist, can make the parser emmit errors. We should consider them optional while trying to parse them in the loop.

This is currently an issue for the distributed runtime, since distributed workers emmit errors while parsing the IR sent by the coordinator.

Example IR that a matrix contains both "sparsity" and "representation" and parsing it emits errors:

func.func @dist(%arg0: !daphne.Matrix<?x403394xf64:sp[2.0816421641779212E-5]:rep[sparse]>, %arg1: !daphne.Matrix<?x?xf64>, %arg2: !daphne.Matrix<?x1xf64>) -> (!daphne.Matrix<?x?xf64>, !daphne.Matrix<?x?xf64>) {
  %0 = "daphne.ewMul"(%arg0, %arg1) : (!daphne.Matrix<?x403394xf64:sp[2.0816421641779212E-5]:rep[sparse]>, !daphne.Matrix<?x?xf64>) -> !daphne.Matrix<?x?xf64:sp[2.0816421641779212E-5]:rep[sparse]>
  %1 = "daphne.maxRow"(%0) : (!daphne.Matrix<?x?xf64:sp[2.0816421641779212E-5]:rep[sparse]>) -> !daphne.Matrix<?x?xf64>
  %2 = "daphne.ewMax"(%1, %arg2) : (!daphne.Matrix<?x?xf64>, !daphne.Matrix<?x1xf64>) -> !daphne.Matrix<?x?xf64>
  %3 = "daphne.ewNeq"(%2, %arg2) : (!daphne.Matrix<?x?xf64>, !daphne.Matrix<?x1xf64>) -> !daphne.Matrix<?x?xf64>
  "daphne.return"(%2, %3) : (!daphne.Matrix<?x?xf64>, !daphne.Matrix<?x?xf64>) -> ()
}

[aristotelis96]

@pdamme @corepointer I would kindly ask for a confirmation from your side that this is the correct way to parse the dialect before merging this.

[pdamme]

By the way, this PR is related to #154 and #155.

[aristotelis96]

Thank you @pdamme for your feedback. I've slightly extended the parser test in order to check IR-parsing after MatrixRepresentationPass, since these changes affect IR produced by that pass.