[BE] Fix MPS build warnings (#83048)
Mostly get rid of unused variables, but also:
- Hide `-[MPSGraphTensorData printNDArray]` behind undefined method
access
- Rename several methods in ScatterGather/TriangularOps:
- `scatterAlongAxisWithDataTensor:`->`scatterAlongAxis:withDataTensor:`
- `gatherAlongAxisWithUpdatesTensor:`->`gatherAlongAxis:withUpdatesTensor:`
- `getCoordinateValueWithShapeTensor:`->`coordinateAlongAxisTensor:withShapeTensor:`
- Add `-Wno-unguarded-availability-new` to suppress 12.3+ availability warnings
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83048
Approved by: https://github.com/albanD
