Support recursive ModuleList / Sequential (#21306)
Summary:
Adds support for recursively compiling `nn.Sequential` and
`nn.ModuleList`. When either is used, it is converted to a
`jit._ConstModuleList` or `jit._ConstSequential` as necessary. Due to
this, we don't need to add it to `__constants__` since it's made
constant on demand.
This PR also moves the recursive script tests out to their own class
`TestRecursiveScript` (the added test is called `test_iterable_modules`)
](https://our.intern.facebook.com/intern/diff/15611738/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21306
Pulled By: driazati
Differential Revision: D15611738
fbshipit-source-id: fac52993990bd2dfad71d044c463a58a3759932a
