[functorch] introduce an experimental map() op. (#88767)
Summary:
We want to introduce an experimental control flow op: map() to export some models as FX graphs correctly.
Some calrification on basic requirements we have in mind:
1. This op can nest cond() and other control flow primitives internally.
2. We don't necessarily need loop carried dependencies for the models we've seen.
3. This map() op can handle dynamically shaped tensor as input and return dynamically shaped output based on input shapes.
4. We should be able to pass through additional arguments to the loop body as extra arguments.
In this diff we introduce a new control flow op `map()` which has the following semantics:
```
def map(f: Callable, xs: Tensor, *args):
# one possible implementation:
return torch.stack([f(x, *args) for x in xs])
```
Test Plan:
pytest functorch/test_control_flow.py
CI
Differential Revision: D41165796
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88767
Approved by: https://github.com/zou3519
