Add script standard library documentation + cleanup (#14912)

Add script standard library documentation + cleanup (#14912)

Summary:
Documents what is supported in the script standard library.

* Adds `my_script_module._get_method('forward').schema()` method to get function schema from a `ScriptModule`
* Removes `torch.nn.functional` from the list of builtins. The only functions not supported are `nn.functional.fold` and `nn.functional.unfold`, but those currently just dispatch to their corresponding aten ops, so from a user's perspective it looks like they work.
* Allow printing of `IValue::Device` by getting its string representation
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14912

Differential Revision: D13385928

Pulled By: driazati

fbshipit-source-id: e391691b2f87dba6e13be05d4aa3ed2f004e31da