[DataLoader] Introduce MapMapDataPipe functional datapipe (#58258)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58258
As part of https://github.com/pytorch/pytorch/issues/57031, this PR adds the `MapMapDataPipe` functional datapipe for the `MapDataPipe` class.
Usage:
```
def fn(x):
return x * 10
dp = CountingDataset(n=10)
dp.map(fn)
```
Reviewed By: ejguan
Differential Revision: D28394510
fbshipit-source-id: 8d71b1f5723dff52385c3ce753944304896af678
