[jit] kill script namespace (#34515)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34515

Once upon a time we thought this was necessary. In reality it is not, so
removing it.

For backcompat, our public interface (defined in `api/`) still has
typedefs to the old `script::` names.

There was only one collision: `Pass` as a `Stmt` and `Pass` as a graph
transform. I renamed one of them.

Test Plan: Imported from OSS

Differential Revision: D20353503

Pulled By: suo

fbshipit-source-id: 48bb911ce75120a8c9e0c6fb65262ef775dfba93

torch/csrc/jit/frontend/parser.cpp

@@ -7,7 +7,6 @@
 
 namespace torch {
 namespace jit {
-namespace script {
 
 Decl mergeTypesFromTypeComment(
     const Decl& decl,
@@ -735,6 +734,5 @@ Expr Parser::parseExp() {
   return pImpl->parseExp();
 }
 
-} // namespace script
 } // namespace jit
 } // namespace torch