[Feature] Dispatch RandomSampler/SliceSampler to without-replacement variants via replacement=False

[vmoens]

Summary

Adds a replacement keyword to the with-replacement sampler classes so users can opt into the without-replacement variant from the simpler constructor:

RandomSampler(replacement=False)               # -> SamplerWithoutReplacement
RandomSampler(replacement=False, drop_last=True, shuffle=False)
SliceSampler(replacement=False, slice_len=5)   # -> SliceSamplerWithoutReplacement
SliceSampler(replacement=False, slice_len=5, drop_last=True)

Implementation: Sampler gains an ABCMeta-derived metaclass _SamplerMeta whose __call__ pops replacement from kwargs and dispatches via a small registry _REPLACEMENT_DISPATCH ({RandomSampler: SamplerWithoutReplacement, SliceSampler: SliceSamplerWithoutReplacement}). When replacement=True (default) the metaclass is a transparent no-op. Samplers that have no without-replacement variant (e.g. PrioritizedSampler) raise TypeError on replacement=False.

The existing SamplerWithoutReplacement / SliceSamplerWithoutReplacement / PrioritizedSliceSampler classes are untouched and remain the canonical entry points; the new keyword is purely additive.

Test plan

• test/test_rb.py::TestSamplers::test_replacement_kwarg_* (6 new tests covering type identity, default forwarding, kwarg forwarding, isinstance invariants, subclass non-interference, error on unsupported variants, and end-to-end use in a ReplayBuffer for both RandomSampler and SliceSampler dispatch).
• Full test/test_rb.py suite passes (3929 passed, no regressions).
• Pickle round-trip confirmed for both dispatch and non-dispatch cases.

🤖 Generated with Claude Code

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/3749

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❗ 1 Active SEVs

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

• Run pull request jobs on OSDC runners in shadow mode

❌ 2 New Failures, 1 Pending

As of commit 5c12071 with merge base cc31dc3 ():

NEW FAILURES - The following jobs have failed:

• Lint / lint-done (gh)
  Process completed with exit code 1.
• Lint / python-source-and-configs / linux-job (gh)
  RuntimeError: Command docker exec -t b80ececbb9d791637cddf9a47e68cbd56b8c14e280078f9ab5e2c87b296ea923 /exec failed with exit code 1

This comment was automatically generated by Dr. CI and updates every 15 minutes.