[caffe2] Support Log1p operator (#54968)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54968
Support Log1p operator to add feature parity with PyTorch.
NumPy doc https://numpy.org/doc/stable/reference/generated/numpy.log1p.html
PyTorch doc https://pytorch.org/docs/stable/generated/torch.log1p.html
Test Plan:
```
$ buck test mode/dev-nosan //caffe2/caffe2/python/operator_test:log1p_op_test
```
Differential Revision: D27422219
fbshipit-source-id: f9eba82bf09c1c440f11a33f8ae2bf8084609457
