Fix contant/placeholder loading in checkGraphCompatibility
Summary: Properly load node inputs as placeholders during onnxifi checkGraphCompatibility only if they are non-weigth inputs to the node
Test Plan:
`buck test glow:`
PASS: 2286
FAIL: 0
SKIP: 456
Reviewed By: jfix71
Differential Revision: D20823088
fbshipit-source-id: 76215b2c0c3934e36714201c7e716e8f95463e6d
