[MPS] Fix GELU for `torch.half` (#86218)
Also, make sure it raises catcheable errors if invoked with integral types
Otherwise, it used to fail with following fatal error invoked for `torch.half` and with similar signatures if invoked for integral types
```
loc("mps_multiply"("(mpsFileLoc): /AppleInternal/Library/BuildRoots/4883e71d-37bd-11ed-b0ef-b25c5e9b9057/Library/Caches/com.apple.xbs/Sources/MetalPerformanceShadersGraph/mpsgraph/MetalPerformanceShadersGraph/Core/Files/MPSGraphUtilities.mm":228:0)): error: input types 'tensor<2xf16>' and 'tensor<1xf32>' are not broadcast compatible
LLVM ERROR: Failed to infer result type(s).
```
Modified `test_gelu_simple` to check both fwd and backward gradients for gelu
