New Runtime pybind API (#6063)

Summary:
Based on this proposal: https://docs.google.com/document/d/10Q4-pt97inQQtFf-FjjwhMaDXXCfk1zGy6V6EkygNUY/edit#heading=h.fcrpnrtb6cud

Historically our pybinding APIs are not following the same C++ modeling
(Program, Method etc) and hence it's hard to use and easy to hit
footguns - for example, if we load the program and return it from a
python method, it goes out of the scope and releases the memory.

This effort is to create Pybind APIs that resembles C++ objects so it's
less confusing to the users.

Add the following python classes:
* `Runtime`: a singleton object hosting methods like `load_program`.
  Returns a `Program` object when calling `load_program`. Also exposes
  the operator registry
* `Program`: each pte file should have one `Program` object. Most
  important method is `load_method` which returns a `Method` object. It
  has a property `method_names` where we can inspect what methods are
  inside this .pte file.
* `Method`: one object per method name in a given `Program`. Exposes
  `execute` which takes in pytree flattened torch tensors as input and
  return pytree flattened output. It also exposes `MethodMeta` for users
  to inspect more information regarding input/output of this method.

Pull Request resolved: https://github.com/pytorch/executorch/pull/6063

Reviewed By: dbort

Differential Revision: D64132360

Pulled By: larryliu0820

fbshipit-source-id: a2f35edc5fd8c200df0812a693e454d66d6a907e