Functools partial support in dynamo (#108846)
The strategy for supporting functools partials is relatively straightforward.
There are 2 cases we need to support:
**1) Functools partials as input**
In this case, we are first seeing the functools partial and it is guaranteed to have a source. As such, the args, keywords, and func of the functools partial are passed through VariableBuilder. As this is the first time we are seeing these objects (as it is an input), we re-enter VariableBuilder with a source referencing the args, keywords, and func as attributes of the input to produce:
- func: A callable VariableTracker (UDF, TorchVariable, etc) depending on the value of `func`
- args: List[VariableTracker] - note, not ListVariableTracker!
- keywords: Dict[str, VariableTracker]
A major benefit of this structure is that it very elegantly matches the args to `call_function`.
We then compose a FunctoolsPartialVariable from the VariableTrackers made above.
**2) Functools partials created within compile**
In this case, we already have all the args as known VTs, and thus just compose a FunctoolsPartialVariable as we do for case (1).
For both (1) and (2) - we propagate all guards from the func, args, and keyword VTs to the FunctoolsPartialVariable
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108846
Approved by: https://github.com/ezyang, https://github.com/jansel
