Dynamo: Fix graph break when iterating over tensor (#94326)
Supports the following with dynamic shapes:
```python
for element in tensor:
# do stuff with element
```
Approach follows what's done when `call_range()` is invoked with dynamic shape inputs: guard on tensor size and continue tracing with a real size value from `dyn_dim0_size.evaluate_expr()`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94326
Approved by: https://github.com/ezyang
