[PJRT] Fix tensor element type for signed integers

[PragmaTwice]

In PJRT, MapElementTypeToMlirType is used in DeviceInstance::TransposeBroadcastDeviceBuffer to generate stablehlo programs for transpose and broadcast tensors.

In the current implementation, IREE_HAL_ELEMENT_TYPE_SINT_N is mapped to MLIR type siN, which is straightforward. But it can produce errors, because in StableHLO ranked tensors of signed integers are not supported (due to a historical reason, I think).

For example, stablehlo.tranpose requires a tensor with constraint HLO_TensorOrPerAxisQuantizedTensor. And HLO_TensorOrPerAxisQuantizedTensor is defined as ranked tensor of many types, including HLO_Int.

Here's the interesting thing: HLO_Int is defined as HLO_SInt or HLO_UInt, and HLO_SInt is defined as signless integers instead of signed integers:

https://github.com/openxla/stablehlo/blob/9e0c9e34d02af255bfe8f896ba7dc910758c6ecf/stablehlo/dialect/Base.td#L39-L43

And above this definition we can see a TODO that states it's due to legacy reasons.

In PJRT plugin MLIR type siN will be used to generate some code like stablehlo.transpose %x : tensor<2x2xsi32>, which is invalid in StableHLO and will lead to a verification error.

ci-exactly: build_packages, test_pjrt