Add nn.ReLU6 to default mapping (#28516)
Summary:
https://discuss.pytorch.org/t/quantized-hard-sigmoid/59013
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28516
Differential Revision: D18128717
Pulled By: jerryzh168
fbshipit-source-id: 4d06d1b54cf9f84a610d79fbadde2c8ef38c33f8
