[BugFix] Allow zero alpha value for PrioritizedSampler

[albertbou92]

Description

The PrioritizedSampler docstring says that the input parameter alpha can be set to 0.0 to obtain uniform sampling. In practice though, a ValueError is raised when alpha = 0.0. This PR changes this to allow for alpha to be 0.0.

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)

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of examples)

Checklist

Go over all the following points, and put an x in all the boxes that apply.
If you are unsure about any of these, don't hesitate to ask. We are here to help!

• 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]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/2164

• 📄 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:

• Upgrade MacOS runner to 14

❌ 5 New Failures, 2 Unrelated Failures

As of commit c7d3bd2 with merge base 767a877 ():

NEW FAILURES - The following jobs have failed:

• Continuous Benchmark (PR) / CPU Pytest benchmark (gh)
  Workflow failed! Resource not accessible by integration
• Continuous Benchmark (PR) / GPU Pytest benchmark (gh)
  Workflow failed! Resource not accessible by integration
• Habitat Tests on Linux / tests (3.9, 12.1) / linux-job (gh)
  RuntimeError: Command docker exec -t 4401afe11b4cb409b87fd3116a1705b23b4307896e991645bc9d81e8b3c057eb /exec failed with exit code 139
• Unit-tests on Linux / tests-cpu (3.10) / linux-job (gh)
  RuntimeError: Command docker exec -t f08d23307d0ec1448588271e589a3565eeb47003963ad59d618a0ec7af99afb4 /exec failed with exit code 1
• Unit-tests on Linux / tests-optdeps (3.10, 12.1) / linux-job (gh)
  RuntimeError: Command docker exec -t b57eabb1ad3c51bd4ecffebd19f3035198b82dfc3412be9b2c87eef72834d904 /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-olddeps (3.8, 11.6) / linux-job (gh) (trunk failure)
  test/test_transforms.py::TestVecNorm::test_state_dict_vecnorm
• Unit-tests on Windows / unittests-cpu / windows-job (gh) (trunk failure)
  test/test_collector.py::TestLibThreading::test_auto_num_threads

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

[vmoens]

Could you add a simple test for this? It could happen that we block it without paying attention

[albertbou92]

done!