[DataLoader] Replacing `traverse` function with `traverse_datapipes` (#85667)
This PR deprecates `traverse` function and replaces it with `traverse_datapipes` instead.
While use `DataLoader`, I realized that it is raising `FutureWarning` even though I am not explicitly using `traverse`. What is happening is that `DataLoader` invokes `traverse(dp, only_datapipe=True)`, and the usage of the keyword causes the `only_datapipe` warning to be raised.
```
/home/ubuntu/miniconda3/lib/python3.8/site-packages/torch/utils/data/graph.py:102: FutureWarning: `only_datapipe` is deprecated from `traverse` function and will be removed after 1.13.
warnings.warn(msg, FutureWarning)
```
A few things we'd like to do:
1. Deprecate the key word arg `only_datapipe`
2. Change the default behavior from `only_datapipe=False` to `only_datapipe=True` in the future
3. Do not raise a warning when users are using the function correctly
This creates a paradox it is impossible for the users to change their code to match the future default behavior (i.e. call `traverse(dp)` without `only_datapipe`):
- they cannot do so because the default behavior of `traverse` hasn't changed yet, so they must use `only_datapipe=True`
- if they use `only_datapipe=True`, eventually the kwarg will go away and cause a runtime error; they also get a `FutureWarning` in the present
IIUC, there doesn't seem to be a way to accomplish those 3 goals without replacing the function with a new one that has a different name; hence, this PR. Let me know if there is a better alternative.
If this looks right, I will send a follow up PR in `TorchData`.
Differential Revision: [D39832183](https://our.internmc.facebook.com/intern/diff/D39832183)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85667
Approved by: https://github.com/ejguan
