Teach pytrees about namedtuple (#62292)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62292
This PR adds pytree support for namedtuples. The challenge about namedtuple
is that each namedtuple class is actually different. This PR does the
following:
- it adds a namedtuple flatten/unflatten. The flatten function returns
a context that is the actual type of the namedtuple subclass. The
unflatten function uses that type to reconstruct the namedtuple
- Special cases all pytree logic to consider all namedtuples the same.
This is done by creating a `_get_node_type(pytree)` helper function that
returns `namedtuple` if `pytree` is any namedtuple subclass. The effect
of this is that all namedtuple subclasses will go through the namedtuple
flatten/unflatten functions
- Adds a `_namedtuple_flatten_spec` function for FX pytrees. This function
flattens the namedtuple based on the spec and is equivalent to the
`_tuple_flatten_spec`.
Test Plan
- new tests in test/test_pytree.py and test/test_fx.py
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D29947302
Pulled By: zou3519
fbshipit-source-id: 19c00665b13546642c315df0f243ad99b8e7ff7c
