[StableHLO] Add conversion for empty op.

[mmanzoorTT]

Stablehlo to TTIR conversion uses a custom type convertor for modifying the unsupported data types to supported data types. tensor::emptyop fails due to not applying this custom type convertor if emptyop is created with unsupported data type.

Sample mlir graph

module {
  func.func @main() -> tensor<1xi1> {
    %0 = tensor.empty() : tensor<1xi1>
    return %0 : tensor<1xi1>
  }
}

Stack dump

aten::empty_strided_0.mlir:4:5: error: failed to legalize unresolved materialization from ('tensor<1xi1>') to 'tensor<1xbf16>' that remained live after conversion	"aten::empty_strided_0.mlir:4:5: error: failed to legalize unresolved materialization from ('tensor<1xi1>') to 'tensor<1xbf16>' that remained live after conversion
    return %0 : tensor<1xi1>
    ^
aten::empty_strided_0.mlir:4:5: note: see current operation: %1 = ""builtin.unrealized_conversion_cast""(%0) : (tensor<1xi1>) -> tensor<1xbf16>
aten::empty_strided_0.mlir:4:5: note: see existing live user here: func.return %1 : tensor<1xbf16>