[FX] Adds PyTree support to FX through `concrete_args` (#55888)
Summary:
```
class Foo(nn.Module):
def __init__(self):
super().__init__()
def forward(self, y, x):
for k in x:
for v in x[k]:
v += y
return x
example_dict = {'x': {'a': [fx.HOLE], 'z': [fx.HOLE, fx.HOLE]}}
new_f = fx.symbolic_trace(Foo(), concrete_args=example_dict)
print(new_f.code)
new_f(torch.randn(5), {'x': {'a': [torch.randn(5)], 'z': [torch.randn(5), torch.randn(5)]}})
fx.symbolic_trace(new_f, concrete_args=example_dict)
```
prints out
```
def forward(self, y, x):
y, tree_2, tree_3, tree_4 = pytree.tree_flatten([y, x])[0]
add = tree_2 + y
add_1 = tree_3 + y
add_2 = tree_4 + y; y = None
return {'a': [tree_2], 'z': [tree_3, tree_4]}
```
Currently, I store `in_spec` as an extra attribute on `fx.Graph`, and then include it when we do the codegen. I'm not sure if this is the right approach - it introduces a divergence between what's in `fx.Graph` and what's in the python code.
Perhaps the best API is something explicit like `fx.Graph.flatten_args`, but that does make calling things a bit ... more verbose.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55888
Reviewed By: jamesr66a
Differential Revision: D27884694
Pulled By: Chillee
fbshipit-source-id: f9e8a70c63a8df63c9f9bd0a6459255daa5a8df8
