add lazy IteratorVariable implementations for map and zip (#131413)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/130750.
Repro of lazy/eager `map` discrepancy without `islice`:
```python
def fn(a, b):
y = 1
def f(x):
nonlocal y
y += 1
return x
l = list(zip([a, b], map(f, [1, 2, 3, 4])))
return a + y
```
The major change is that we implement `MapVariable` and `ZipVariable` based on `IteratorVariable`. Before, `map` and `zip` were being traced by immediately unpacking the result as a `TupleVariable`, which is wrong in cases such as the example above.
`MapVariable`s are not allowed to be unpacked while `ZipVariable`s can only be unpacked if all of its iterables can also be unpacked.
We also add new `[has_]force_unpack_var_sequence` methods to `VariableTracker` for the case where it is safe to unpack the entire sequence lazily, e.g., when building a list from a map (i.e. `list(map(f, ...))`).
X-link: https://github.com/pytorch/pytorch/pull/131413
Approved by: https://github.com/anijain2305
Reviewed By: clee2000
Differential Revision: D60322948
Pulled By: williamwen42
fbshipit-source-id: 52f7763d58943696c0ed2abf8fa03fa6795d1be9
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3f8fa499 - add lazy IteratorVariable implementations for map and zip (#131413)
