Document `torch.jit.is_tracing()` (#67326)
Summary:
This PR adds `torch.jit.is_tracing()` to the JIT API reference.
This function is widely used but left undocumented: https://github.com/search?q=torch.jit.is_tracing&type=code
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67326
Reviewed By: tugsbayasgalan
Differential Revision: D31985251
Pulled By: Krovatkin
fbshipit-source-id: 852b432b08d63df8bd7a7a02c9555e61f5f37978
