The Pile's _FEATURE spec seems to be incorrect
## Describe the bug
If you look at https://huggingface.co/datasets/the_pile/blob/main/the_pile.py:

For "all"
*  the pile_set_name is never set for data
* there's actually an id field inside of "meta" 

For subcorpora pubmed_central and hacker_news:
* the meta is specified to be a string, but it's actually a dict with an id field inside.

## Steps to reproduce the bug


## Expected results
Feature spec should match the data I'd think?

## Actual results
Specify the actual results or traceback.

## Environment info
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