[BugFix] Document AtariDQN dataset unavailability

[YeonwooSung]

Description

The public GCS source for AtariDQNExperienceReplay (gs://atari-replay-datasets/dqn/...) has been removed. vmoens confirmed this on #3113; gsutil now fails with AccessDenied, while the class docstring still presented download as the happy path.

• Document that the hosted dataset is gone. Examples that would download are marked # doctest: +SKIP and take a local copy under root.
• On AccessDenied / gsutil failure, raise a RuntimeError that the hosted dataset was removed and that root= should point at a local copy. Do not surface a raw AccessDenied as the user-facing error.
• Skip live download tests in test/libs/test_datasets.py (reason points at [BUG] AtariDQNExperienceReplay shows AccessDeniedException #3113). Add offline tests for the AccessDenied error path (mocked gsutil, no network) and for constructing from data already on disk.

The class stays importable. Existing local copies under root still load.

Code example

from torchrl.data.datasets import AtariDQNExperienceReplay

# A missing local copy no longer yields a raw GCS AccessDenied.
try:
    AtariDQNExperienceReplay("Pong/5", batch_size=4, root="/tmp/atari", download=True)
except RuntimeError as err:
    assert "no longer available" in str(err)
    assert "root=" in str(err)

# A previously downloaded copy under root still loads.
dataset = AtariDQNExperienceReplay(
    "Pong/5", batch_size=128, root="/path/to/atari", download=False
)

Motivation and Context

Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.
You can use the syntax close #15213 if this solves the issue #15213

• I have raised an issue to propose this change (required for new features and bug fixes)

close #3113

Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• Documentation (update in the documentation)

Checklist

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• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.

[pytorch-bot]

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❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• linux.dgx.b200 runner queuing exceeds 4 hours and dgxb200-05 "CUDA not available"

❌ 2 New Failures, 2 Unrelated Failures

As of commit e4c2d46 with merge base 84d4602 ():

NEW FAILURES - The following jobs have failed:

• Lint / lint-done (gh)
• Lint / python-source-and-configs / linux-job (gh)
  RuntimeError: Command docker exec -t 955306a1b0813354d0742ddc4a904649cbe22f3e419b83ec5885e81c79b6d3de /exec failed with exit code 1

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Unit-tests on Linux / tests-gpu (3.12, 13.0, 2) / linux-job (gh) (trunk failure)
  test/test_inference_server.py::TestProcessSlotTransport::test_server_batched_pass_on_cuda[True]
• Unit-tests on Linux / tests-stable-gpu (3.12, 13.0, 2) / linux-job (gh) (trunk failure)
  test/test_inference_server.py::TestProcessSlotTransport::test_server_batched_pass_on_cuda[True]

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