[fx2trt] Add acc_ops.sign and converter for it (#63876)

[fx2trt] Add acc_ops.sign and converter for it (#63876)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63876

Add `acc_ops.sign` which maps from `torch.sign`.

Add a plugin (not support dynamic shape currently) for `acc_ops.sign`. The plugin calls `at::sign` directly.

Test Plan: buck test mode/opt -c python.package_style=inplace -c fbcode.nvcc_arch=a100 caffe2/torch/fb/fx2trt:test_unary_ops

Reviewed By: yinghai

Differential Revision: D30518081

fbshipit-source-id: a0b9e6c30deac0b04b8cb09a162579e229985330