Misc pickler improvements (#19638)
Summary:
Stack from [ghstack](https://github.com/ezyang/ghstack):
* **#19638 [jit] Serialize attribute module as torch.jit._pickle**
* use `torch.jit._pickle` as the module for globals in the pickle program. Pickle will try to resolve these to the actual functions in `torch.jit._pickle.py` automatically (I believe this can also be overridden to point to whatever functions you want). This means that `pickle.load("my_model/attributes.pkl")` will work instead of having to use a custom `pickle.Unpickler`
* use `REDUCE` opcodes instead of `BUILD` to make use of the last bullet
* use a union in the unpickler to support globals better (+ any future metadata we might need that can't be stored in an `IValue`), this makes some of the code around `IntList`s clearer and lets us get rid of any lookbehind for opcodes
* pickle things as a tuple instead of a list (an immutable result is more semantically correct)](https://our.intern.facebook.com/intern/diff/15111203/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19638
Pulled By: driazati
Differential Revision: D15111203
fbshipit-source-id: 526c6c2b63a48eb1cba1c658045a7809730070dd
