[Static Runtime] Make module_ optional in StaticModule (#65882)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65882
`torch::jit::Module` is refcounted. There is no need to wrap it in a `shared_ptr`.
Test Plan:
```
buck run //caffe2/benchmarks/static_runtime:static_runtime_cpptest
```
Reviewed By: mikeiovine
Differential Revision: D31012222
fbshipit-source-id: 74d234bd85423e5ba0e396f24899631354a2c74b
