[jit] Add lazy script decorator (#34935)
Summary:
Stacked PRs
* #34938 - [jit] Remove stray `script`
* **#34935 - [jit] Add lazy script decorator**
Some users maintain libraries of code that is largely trace-able but not
script-able. However, some functions may need to be `torch.jit.script`ed if
they contain control flow so the tracer will use the compiler version.
This however impacts library start up time as in #33418, so this PR adds
a workaround in the form of a `torch.jit._lazy_script_while_tracing`
that will only initialize the compiler if the function is called while
actually tracing.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34935
Pulled By: driazati
Differential Revision: D20569778
fbshipit-source-id: d87c88c02b1abc86b283729ab8db94285d7d4853
