trunk/7659b78f44ac1098acb26e733e76162b02a6c4d1: [MPS] Migrate sigmoid_backward from MPSGraph to Metal (#187151)

Replace the MPSGraph-based sigmoid_backward_out_mps with a native Metal kernel registered through the shared sigmoid_backward_stub.

Complex types are handled via the conjugated derivative (grad * conj((1 - output) * output)), matching CUDA's behavior.

Microbenchmark (us per call, A/B from the same script with only the impl

swapped):

shape|dtype                      mpsgraph     metal   speedup

1024|float32                        12.81      7.43     1.72x

1024|float16                        14.82      7.10     2.09x

1024|bfloat16                       12.11      7.15     1.69x

1024x1024|float16                   18.56     14.67     1.26x

1024x1024|float32                   45.02     40.06     1.12x

8x1024x1024|float32                382.21    376.67     1.01x

32x1024x1024|float32              1487.84   1493.75     1.00x

Metal wins on dispatch overhead at small sizes; parity at large sizes where

the op is memory-bandwidth bound, as expected for ~3 FLOPs/element.

Co-Authored-By: Claude Opus 4.7 [email protected]

Pull Request resolved: #187151

Approved by: https://github.com/Skylion007