[BugFix] Fix SliceSampler for torch.compile compatibility

[vmoens]

Stack from ghstack (oldest at bottom):

• [Feature] Add _Contiguous module and reshape improvements to encoders/decoders #3306
• -> [BugFix] Fix SliceSampler for torch.compile compatibility #3298

Convert seq_length to tensor before torch.where operations to avoid
torch.compile inductor C++ codegen bugs with mixed scalar/tensor int64
in blendv operations.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/3298

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⏳ 1 Pending, 1 Unrelated Failure

As of commit 9415b1d with merge base d7ef78b ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Unit-tests on Windows / unittests-cpu (3.10, windows.4xlarge, cpu) / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.

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