[MPS] Facial Landmark Detection doesn't work on MPS

[Christywl]

Test Env:
Chromium Version: nightly build 75.0.3739.0 (9642fde)
Platform: macOS

Expected Result:
Facial Landmark Detection should work.

Actual Result:
After the crash issue #716 was fixed, Facial Landmark Detection doesn't work on MPS:

How to Reproduce:

1. Launch chromium and visit:
   https://intel.github.io/webml-polyfill/examples/facial_landmark_detection/
2. Select SUSTAINED_SPEED
3. Select one model:
   • SSD MobileNet v1
   • SSD MobileNet v2
   • SSDLite MobileNet v2

[fujunwei]

It's not regression but relieving symptoms after fixing crashed issue.

A node is optimized by last graph that isn't result node, so the value of temporary_mps_images[input_index] will be null, the model need to be supported above mac os 10.15 with resultImages

init?(device: MTLDevice,
resultImages: [MPSNNImageNode],
resultsAreNeeded areResultsNeeded: UnsafeMutablePointer?)