Fixes pytorch#105203

Facing a similar problem to the linked issue, where variable sized input data can mean that a handful of slow to process samples holds up smaller and faster to process samples from being used. This also leads to lower GPU utilization as well. In certain cases, e.g. evaluation epochs, inference pipelines or other cases where reproducibility isn't important, this can bring significant speed ups.

This PR adds an `allow_out_of_order` bool input to the `DataLoader` class, defaulting to `false`, which when set to `true` will returning data from workers in whatever order they are ready/processed in, rather in the strict index order.
Instead of storing data that was returned out of order, it is passed directly to the main thread and the entry in `_task_info` is deleted. The main changes are they to check that an entry in `_task_info` does exist, and only increasing `self._rcvd_idx` when the lowest index remaining gets returned.

Two tests are added to test this for iterable type datasets and index type datasets.

Pull Request resolved: pytorch#141833
Approved by: https://github.com/andrewkho

Fixes #105203 and is a follow up PR to #141833

When `in_order` is True (the default), tasks are given out to workers in a round robin fashion. When `in_order` is False this is no longer needed, as we give up guarantees of reproducibility, and instead tasks should be given to workers that are able to perform work.
In this PR I've added tracking of the number of outstanding tasks for each worker (updated when tasks are added to their queue, and when data is returned to the main thread). When finding the next queue to add a task to, if `in_order` is False it will only add the task to the workers queue if it has fewer than `_prefetch_factor` tasks outstanding.
The current default behaviour is left as is.

Tests are also updated to assert on the worker IDs for each sample of data returned.
I've run the following to confirm they aren't flaky
```bash
for i in {1..20}; do python test/test_dataloader.py TestOutOfOrderDataLoader; done
```

Pull Request resolved: #142324
Approved by: https://github.com/andrewkho