Fix unclosed aiohttp.ClientResponse objects

[Wauplin]

Fix #2521.

Follow-up PR after #2496. Closing sessions when exiting the AsyncInferenceClient is not enough to get rid of all warnings. The ClientResponse objects also have to be closed explicitly when in case they are not consumed entirely. This PR fixes this by registering all pending responses that have to be closed when a ClientSession is closed.

It was a transient error depending on when the garbage collector was triggered, making it difficult to investigate. Many thanks to @tomjorquera who provided a reproducible example. I used it to run over 100 examples (with a progress bar) and it seems resolved:

# on 'main' branch
  3%|██▏                                                                    | 3/100 [00:02<01:08,  1.42it/s]Unclosed connection
client_connection: Connection<ConnectionKey(host='api-inference.huggingface.co', port=443, is_ssl=True, ssl=None, proxy=None, proxy_auth=None, proxy_headers_hash=4764392330092113202)>
 19%|█████████████▎                                                        | 19/100 [00:12<00:49,  1.63it/s]Unclosed connection
client_connection: Connection<ConnectionKey(host='api-inference.huggingface.co', port=443, is_ssl=True, ssl=None, proxy=None, proxy_auth=None, proxy_headers_hash=4764392330092113202)>
Unclosed connection
client_connection: Connection<ConnectionKey(host='api-inference.huggingface.co', port=443, is_ssl=True, ssl=None, proxy=None, proxy_auth=None, proxy_headers_hash=4764392330092113202)>
Unclosed connection
client_connection: Connection<ConnectionKey(host='api-inference.huggingface.co', port=443, is_ssl=True, ssl=None, proxy=None, proxy_auth=None, proxy_headers_hash=4764392330092113202)>
 29%|████████████████████▎                                                 | 29/100 [00:18<00:46,  1.54it/s]
(...)

# with this PR
 100%|█████████████████████████████████████████████████████████████████████| 100/100

I've also added a test to ensure responses are closed when a session is closed. (and thanks to #2521, the sessions are closed when the AsyncInferenceClient is closed)

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[hanouticelina]

thanks @Wauplin! before this PR, I thought that the ClientResponse objects are closed implicitly when closing the ClientSession, but it seems that aiohttp allows streaming responses to outlive sessions. Nice 👍

[Wauplin]

Same here, I wouldn't have expected them to outlive sessions. Most of the time, those objects are meant to be used in context managers which we don't do here for practical reasons. This is why we are facing those corner cases.
(btw, using httpx for both InferenceClient and AsyncInferenceClient would have been much easier to deal with instead of requests+aiohttp. We've not done it for consistency with other parts of the lib' but we can think of a switch in the future if we really see a need).

In the meantime, let's get this merged :)