[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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❌ 13 New Failures, 2 Cancelled Jobs, 2 Unrelated Failures

As of commit 55d08ab with merge base df00d61 ():

NEW FAILURES - The following jobs have failed:

• Continuous Benchmark (PR) / CPU Pytest benchmark (gh)
  Process completed with exit code 1.
• Habitat Tests on Linux / tests (3.9, 12.8) / linux-job (gh)
  RuntimeError: Command docker exec -t d1ffefc9f0aeb56e588b05f64c8ae435b788ff0954b2c75bb55b6e9c50ab2f00 /exec failed with exit code 1
• Libs Tests on Linux / unittests-gym (3.9, 12.8) / linux-job (gh)
  RuntimeError: Command docker exec -t 63bfe3c950ed9e03574db1a4b676186c077480227d04808a95b957f9a692c871 /exec failed with exit code 1
• Unit-tests on Linux / tests-cpu (3.10) / linux-job (gh)
  test/test_transforms.py::TestRayModuleTransform::test_ray_module_transform_scheme_collector
• Unit-tests on Linux / tests-cpu (3.11) / linux-job (gh)
  test/test_transforms.py::TestRayModuleTransform::test_ray_module_transform_scheme_collector
• Unit-tests on Linux / tests-cpu (3.12) / linux-job (gh)
  test/test_transforms.py::TestRayModuleTransform::test_ray_module_transform_scheme_collector
• Unit-tests on Linux / tests-cpu (3.13) / linux-job (gh)
  test/test_transforms.py::TestRayModuleTransform::test_ray_module_transform_scheme_collector
• Unit-tests on Linux / tests-gpu (3.12, 13.0) / linux-job (gh)
  test/test_transforms.py::TestRayModuleTransform::test_ray_module_transform_scheme_collector
• Unit-tests on Linux / tests-gpu-distributed (3.12, 13.0) / linux-job (gh)
  test/test_distributed.py::TestRayCollector::test_ray_collector_policy_constructor
• Unit-tests on Linux / tests-olddeps (3.9, 11.6) / linux-job (gh)
  test/test_transforms.py::TestRayModuleTransform::test_ray_module_transform_scheme_collector
• Unit-tests on Linux / tests-optdeps (3.12, 13.0) / linux-job (gh)
  test/test_transforms.py::TestRayModuleTransform::test_ray_module_transform_scheme_collector
• Unit-tests on Linux / tests-stable-gpu (3.12, 13.0) / linux-job (gh)
  test/test_transforms.py::TestRayModuleTransform::test_ray_module_transform_scheme_collector
• Unit-tests on Linux / tests-stable-gpu-distributed (3.12, 13.0) / linux-job (gh)
  test/test_distributed.py::TestRayCollector::test_ray_collector_policy_constructor

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• Continuous Benchmark (PR) / GPU Pytest benchmark (gh)
  ##[error]The operation was canceled.
• SOTA Tests on Linux / tests (3.10, 13.0) / linux-job (gh)
  ##[error]The operation was canceled.

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-cpu (3.14) / linux-job (gh) (trunk failure)
  test/test_transforms.py::TestConditionalSkip::test_trans_parallel_env_check[False-False]
• Unit-tests on Windows / unittests-cpu (3.10, windows.4xlarge, cpu) / windows-job (gh) (trunk failure)
  test/test_transforms.py::TestConditionalSkip::test_trans_parallel_env_check[False-False]

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