[dynamo] inlining into __iter__ of user defined object (#119243)
Fixes #119198.
This PR make dynamo inline `__iter__` of a user defined object instead of creating a graph break. Also added a new test, which shows:
1. the loop is unrolled
2. the length of the loop is guarded when inlining `__iter__`
```python
class Mod:
def __init__(self):
self.a = [torch.randn(2, 2), torch.randn(2, 2)]
def __iter__(self):
return iter(self.a)
def f(mod):
ret = []
for x in mod:
ret.append(x + 1)
return ret
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119243
Approved by: https://github.com/jansel
