[fx] Separate globals assignment from code generation (#51974)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51974
Right now, when an FX `Graph` references an external object, we will emit
code like:
import foo
def forward(input: foo.bar.baz):
...
This is problematic in a world with `torch.package`, since then name
`foo.bar.baz` may reference a name from any number of packages.
This PR lays the groundwork for FX-package integration by separating the
resolution of external references from the genration of the function
code.
When generating a Graph's Python source, we keep track of all external
references and assign them unique names. At the end, we have a
dictionary mapping names -> actual objects. This becomes the `globals`
namespace we pass to `exec` when installing the forward function in a
`GraphModule`. This is nice because we can always be sure that `exec` is
seeing the same objects that were referenced from the `Graph`, no import
statements needed.
At serialization time, we use a `ModuleEnv` to resolve the globals dict
to a set of import statements that can be run to reprodce the `global`
namespace. This is only used on serialiation/deserialization, and those
functions are expected to check that the import statements are producing
the correct results.
Concretely, the code above will now look like:
from foo.bar import baz as foo_bar_baz
def forward(input: foo_bar_baz):
...
Test Plan: Imported from OSS
Reviewed By: jamesr66a
Differential Revision: D26340593
Pulled By: suo
fbshipit-source-id: fe247f75205d0a03fd067bdd0f95491e8edf1436
