[MHLO] Lowers PadOp (constant mode) & GatherElements Op to MHLO

[chenchongsong]

This pull request lowers the following ops to MHLO:

1. Pad op with mode as 'constant'
2. GatherElements op

[jenkins-droid]

Can one of the admins verify this patch?

[tungld]

@jenkins-droid test this please

[AlexandreEichenberger]

FYI, on Windows, they treat many warnings as errors. This might be a simple fix... from the error logs

D:\a\1\onnx-mlir\third_party\mlir-hlo\stablehlo\stablehlo\dialect\Base.cpp(392): warning C4927: illegal conversion; more than one user-defined conversion has been implicitly applied
D:\a\1\onnx-mlir\third_party\mlir-hlo\stablehlo\stablehlo\dialect\Base.cpp(392): note: while calling the constructor 'mlir::FailureOr<mlir::ShapedType>::FailureOr(T &&)'
        with
        [
            T=mlir::ShapedType
        ]

[AlexandreEichenberger]

@chenchongsong I believe the error should be an easy one to fix.

[Connor-XY]

I will help with this one

[AlexandreEichenberger]

Should this PR be closed because functionality has been taken over by #2602?

[Connor-XY]

Should this PR be closed because functionality has been taken over by #2602?

@AlexandreEichenberger Yes, please close this if possible. Chongsong is away and may not be able to close it promptly.