add qnnpack path for hardtanh (#35779)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35779
Adds a QNNPack path for the clamp kernel, which is useful for
hardtanh.
Test Plan:
python test/test_quantized.py TestQNNPackOps.test_hardtanh
Imported from OSS
Differential Revision: D20778588
fbshipit-source-id: 537de42e795a9c67924e1acb1d33b370beb9dbf5
