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Differential Revision: [D38262867](https://our.internmc.facebook.com/intern/diff/D38262867)

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Differential Revision: [D38262867](https://our.internmc.facebook.com/intern/diff/D38262867)

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Changes:
* Imposing the single iterator constraint on `DataLoader2` by having its `__iter__` method returns a wrapper that can track if an iterator is valid or not
* Previously, `__iter__` returns `self`, but now it will return an instance of `_DataLoader2Iterator`
* Aside from the behavior related to reset (see below), we expect other behaviors to stay the same.

Prior behavior:
```python
dl = DataLoader2(IterableWrapper(range(10)))
it1 = iter(dl)
print(next(it1))  # 0
it2 = iter(dl)  # No reset here
print(next(it2))  # 1
print(next(it1))  # 2
```

New behavior with this PR:
```python
dl = DataLoader2(IterableWrapper(range(10)))
it1 = iter(dl)
print(next(it1))  # 0
it2 = iter(dl)  # DataLoader2 resets with the creation of a new iterator
print(next(it2))  # 0
print(next(it1))  # Raises exception, since it1 is no longer valid
```

Differential Revision: [D38262867](https://our.internmc.facebook.com/intern/diff/D38262867)

[ghstack-poisoned]

Changes:
* Imposing the single iterator constraint on `DataLoader2` by having its `__iter__` method returns a wrapper that can track if an iterator is valid or not
* Previously, `__iter__` returns `self`, but now it will return an instance of `_DataLoader2Iterator`
* Aside from the behavior related to reset (see below), we expect other behaviors to stay the same.

Prior behavior:
```python
dl = DataLoader2(IterableWrapper(range(10)))
it1 = iter(dl)
print(next(it1))  # 0
it2 = iter(dl)  # No reset here
print(next(it2))  # 1
print(next(it1))  # 2
```

New behavior with this PR:
```python
dl = DataLoader2(IterableWrapper(range(10)))
it1 = iter(dl)
print(next(it1))  # 0
it2 = iter(dl)  # DataLoader2 resets with the creation of a new iterator
print(next(it2))  # 0
print(next(it1))  # Raises exception, since it1 is no longer valid
```

Differential Revision: [D38262867](https://our.internmc.facebook.com/intern/diff/D38262867)

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